Story highlights NATO releases images showing tank presence near Russia's border with Ukraine

Security service says a homemade bomb was left outside gate of President's offices

Three Ukrainian soldiers killed in attack by separatists

Ukraine's President vows punishment for plane shootdown

European leaders Saturday held telephone talks with Russian President Vladimir Putin and his Ukrainian counterpart after a Ukrainian military plane was shot down by separatists, killing all 49 people on board, officials said.

The Ilyushin-76 military transport plane went down early Saturday while approaching an airport in the eastern Ukrainian city of Luhansk, said military spokesman Vladislav Seleznyov from the anti-terror operation unit.

Ukraine's defense ministry said the plane, which was transporting military personnel, was shot down by insurgents using anti-aircraft machine guns.

The attack, which represents one of the bloodiest single events in that nation's current period of turmoil, spurred a diplomatic flurry to limit further escalation of the crisis.

On Saturday, French President Francois Hollande and German Chancellor Angela Merkel held a telephone discussion with Putin on the crisis in Ukraine, according to the Elysee Palace and the Kremlin.

Hollande and Merkel expressed their deep concern about the ongoing fighting in eastern Ukraine, particularly Saturday's attack, the Elysee Palace said. The French and German leaders stressed the importance of reaching a cease-fire in Ukraine and avoiding the transfer of fighters and weapons across the Ukrainian-Russian border, according to Hollande's office.

Where unrest has occurred in E. Ukraine

Photos: Photos: Ukraine after the election Photos: Photos: Ukraine after the election Ukraine after the election – Ukrainian soldiers sit on an armored vehicle as they take up a position in a sunflower field near Donetsk, Ukraine, on Thursday, July 10. Here's a look at the upheaval that has persisted in eastern Ukraine since the election of President Petro Poroshenko. Hide Caption 1 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A man rides a bike past the tail of a bombshell outside Slovyansk, Ukraine, on July 10. Hide Caption 2 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Eugenia Gubareva cries on July 10 after finding clothes belonging to her parents, who were killed during a bomb shelling in Mikolaivka, Ukraine. Hide Caption 3 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A man cleans up debris in his apartment after a shelling in Slovyansk on July 10. Hide Caption 4 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – People walk under a destroyed railroad bridge near the village of Novobakhmutivka on Monday, July 7. Hide Caption 5 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A woman cries as her house burns after a bombing in Slovyansk on Monday, June 30. Hide Caption 6 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – People say goodbye to volunteer soldiers in Kiev, Ukraine, before they leave for the eastern part of the country to join the ranks of a special battalion on Monday, June 23. Hide Caption 7 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – People take refuge in a bomb shelter during clashes between Ukrainian troops and pro-Russia fighters in Slovyansk on Sunday, June 22. Hide Caption 8 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Riot police surround the Kiev Pechersk Lavra, an Orthodox Christian monastery in Kiev where radical masked activists gather to protest against separatists on June 22. Hide Caption 9 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Ukrainian President Petro Poroshenko awards wounded soldiers with state awards in a military hospital in Kiev on Saturday, June 21. Hide Caption 10 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Debris lies scattered at an airport in Luhansk, Ukraine, on Saturday, June 14, after the crash of a Ukrainian Ilyushin-76 military transport plane. A military spokesman said the aircraft was shot down by pro-Russian separatists, killing all 49 aboard. Hide Caption 11 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A pro-Russian fighter collects ammunition from the debris of the downed military aircraft on June 14. Hide Caption 12 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Pro-Russian fighters walk past the wreckage of the downed aircraft. Hide Caption 13 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A military vehicle was destroyed during a clash between Ukrainian troops and pro-Russian separatists Friday, June 13, in Mariupol, Ukraine. Hide Caption 14 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Locals in Mariupol gather around a burning military vehicle at the site of a battle between Ukrainian troops and pro-Russian fighters on June 13. Hide Caption 15 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A pro-Russian fighter in Slovyansk checks a woman's documents as she leaves the city on Thursday, June 12. Hide Caption 16 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A pro-Russian fighter stands guard at a checkpoint in Slovyansk on June 12. Hide Caption 17 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A girl looks out of a bus window as she leaves Slovyansk on Monday, June 9. Hide Caption 18 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Poroshenko holds the ceremonial mace during his inauguration ceremony Saturday, June 7, in Kiev. Poroshenko was elected three months after the ouster of former President Viktor Yanukovych. Hide Caption 19 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A Ukrainian tank opens fire during a battle with pro-Russian separatist fighters in Slovyansk on Friday, June 6. Hide Caption 20 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A Ukrainian soldier prepares explosives during a battle with pro-Russian separatists in Slovyansk on June 6. Hide Caption 21 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A Ukrainian soldier observes the road at a checkpoint outside of Amvrosiivka, Ukraine, on Thursday, June 5. Hide Caption 22 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Ukrainian soldiers take position during a battle with pro-Russian separatists outside Slovyansk on June 5. Hide Caption 23 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Pro-Russian militants stand guard at a barricade and checkpoint in Donetsk on Sunday, June 1. Hide Caption 24 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A Ukrainian soldier shoots a grenade launcher during a battle with pro-Russian separatists in Slovyansk on Saturday, May 31. Hide Caption 25 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A protester from Kiev's Independence Square gestures May 31 as fellow protesters burn tires to protect their barricades from being dismantled by communal services. Hide Caption 26 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A Ukrainian armored personnel carrier takes position during a battle with pro-Russian separatist fighters May 31 in Slovyansk. Hide Caption 27 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Ukrainian soldiers rest at a checkpoint outside Slovyansk on Thursday, May 29. Hide Caption 28 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Smoke rises from a shot-down Ukrainian Army helicopter outside Slovyansk on May 29. Hide Caption 29 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A man stands in front of a truck that was bombed by Ukrainian soldiers during clashes with armed rebels Tuesday, May 27, in Donetsk. Hide Caption 30 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Medical staff prepare to clean the body of a pro-Russian militant at the Kalinina morgue in Donetsk on May 27. Hide Caption 31 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – The body of a pro-Russian militant lies on a stretcher at a morgue in Donetsk on May 27. He was killed in clashes around Donetsk's airport, which was seized by pro-Russian separatists a day earlier. Ukrainian forces moved in and reclaimed the facility. Hide Caption 32 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Firefighters work on extinguishing a fire at a local sports hall in Donetsk on May 27. Hide Caption 33 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – People pile up sandbags to set up a shooting position on the road leading from the Donetsk airport on May 27. Hide Caption 34 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A man dressed in a prisoner costume takes part in a protest against government corruption May 27 in Kiev. Hide Caption 35 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Pro-Russian militants take position on the roof of Donetsk International Airport on Monday, May 26. Hide Caption 36 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A pro-Russian militant takes position on the roof of the Donetsk airport on May 26. Hide Caption 37 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A man runs for cover during fighting around the airport on May 26. Hide Caption 38 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Smoke rises from the airport on May 26 after an airstrike by the Ukrainian military. Hide Caption 39 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Pro-Russian gunmen take positions near the airport on May 26. Hide Caption 40 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Men stand next to pro-Russian militants as a woman runs away during clashes near the airport on May 26. Hide Caption 41 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A pro-Russian militant guards a checkpoint on May 26, blocking a highway that links the Ukrainian city of Kharkiv with Rostov-on-Don, Russia. Hide Caption 42 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A pro-Russian militant teaches recruits how to use a machine gun in Senyonovka, Ukraine, on May 26. Hide Caption 43 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A woman walks May 26 near barricades built by protesters in Kiev's Independence Square. Vitali Klitschko, Kiev's future mayor and a former boxing champion, promised to dismantle the iconic protest encampment that helped oust Yanukovych but now clogs traffic and draws public complaints. Hide Caption 44 of 50 Photos: Photos: Ukraine after the election Ukraine under new elected leadership – Ukraine's newly elected president, Petro Poroshenko, talks alongside Klitschko, right, during a news conference in Kiev on May 26. Poroshenko, a billionaire candy tycoon known as the "Chocolate King," is a seasoned politician known for his pro-European Union views. Hide Caption 45 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Election commission officials count ballots at a polling station in Kiev on Sunday, May 25. Hide Caption 46 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – Artist Ivan Voronov, 91, and his wife, Svetlana Samoilechenko, 86, cast votes from their Kiev home on May 25. Hide Caption 47 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – An elderly woman casts her vote May 25 as a mobile Ukrainian election committee visited Orane, a village north of Kiev. Hide Caption 48 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A woman holds the hand of a pro-Russian gunman in Donetsk's Lenin Square on May 25. A large separatist rally was held in Donetsk around lunchtime. The protesters chanted pro-Russian slogans as they were addressed by separatist leaders. Hide Caption 49 of 50 Photos: Photos: Ukraine after the election Ukraine after the election – A man prepares a ballot at a polling station in the eastern Ukrainian town of Dobropillya on May 25. See the crisis in Ukraine before the election Hide Caption 50 of 50

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The Kremlin said the three leaders also talked about the drafting of a United Nations Security Council resolution aimed at promoting a settlement. The European leaders stressed the need for an agreement on continuing Russian natural gas deliveries to Ukraine, according to the Elysee.

"Situation is dangerous, but I hope will remain under control," Ukrainian Deputy Foreign Minister Danylo Lubkivsky told CNN Saturday. "With no doubt, more pressure on Russia is needed."

In a phone conversation with Hollande, Ukrainian President Petro Poroshenko urged the European Union to immediately consider sanctions against Russia, including the termination of "military-technical cooperation," Poroshenko's office said.

Ukrainian Prime Minister Arseniy Yatsenyuk spoke by phone with U.S. Secretary of State John Kerry, who expressed condolences for the loss of life and "strong concern" about the flow of heavy weapons and militants across the border from Russia, a senior State Department official said.

Kerry also talked with Russian Foreign Minister Sergey Lavrov, voicing his concern over the downing of the Ukrainian transport plane and pressing Lavrov to "make clear" Russia's commitment to peace, a cease-fire and political dialogue, the state department official said.

In a statement, Poroshenko said the army and state border service had closed 248 kilometers of Ukraine's border.

"The goal us to take back control of Ukraine over the state border and provide adequate response to terrorists," he said.

A U.S. National Security Council spokeswoman Saturday condemned that attack, saying officials were "deeply concerned about the situation in eastern Ukraine, including by the fact that militant and separatist groups have received heavy weapons from Russia, including tanks, which is a significant escalation."

"Regarding sanctions, we have said that we want to give Russia the opportunity to pursue de-escalation," NSC spokeswoman Laura Lucas Magnuson said in the statement. "President Obama made clear that de-escalation depends upon Russia ceasing support for separatists in eastern Ukraine and stopping the provision of arms and materiel across the border, and we urge Russia to do so."

Video posted to YouTube claimed to show the plane being shot down. Small flashes can be seen climbing skyward, followed by large flashes on the ground on the distant horizon.

The plane was also carrying military machinery and supplies, the ministry said. Officials originally had said the incident occurred late Friday but later updated the time.

Poroshenko announced a day of mourning Sunday to honor those killed and said Ukrainians are grieving.

"It is a great loss not only for the families of the deceased ones, but for the whole country," he said. "Everyone involved in this cynical, large-scale terrorist act will be punished."

In an indication of the continued opposition from some quarters to the Kiev government, the state security office said in a statement Saturday that a homemade bomb was found overnight outside a gate to Poroshenko's office.

The device was made of grenades and a kilogram of other material, such as metal screws, and had a threatening note attached that demanded that the President stop military action against the separatists, the statement said. The person who left it was spotted but fled without detonating it.

The Kiev-based government is carrying out what it calls an anti-terrorist operation, centered in the Donetsk and Luhansk regions, to try to regain control from pro-Russia separatists.

Russia and Ukraine have been engaged in a tense standoff since March, when Russia annexed Crimea and massed troops along its border with Ukraine. Moscow has since begun to withdraw those forces.

NATO: Tank images raise questions

NATO released images Saturday that it said raised "significant questions concerning Russia's ... involvement in the movement of military equipment from Russian territory into Ukraine."

Ukraine's acting Interior Minister Arsen Avakov claimed Thursday that three Russian tanks had crossed the border into Ukraine as part of a larger armored column, prompting a skirmish between Ukrainian and Russian forces. The vehicles entered at a checkpoint controlled by separatists in Ukraine's eastern Luhansk region, Avakov said, citing Ukrainian intelligence.

The Russian Foreign Ministry told the BBC on Thursday that the claim its tanks had crossed the border was "another fake piece of information."

But the United States also believes tanks and heavy weapons have crossed into Ukraine from Russia after moving from a deployment site in southwest Russia, State Department spokeswoman Marie Harf said Friday.

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The images released by NATO show the movements of Russian forces at a site near Russia's southwestern city of Rostov-on-Don, about 75 kilometers from a border crossing the body says is controlled by Luhansk separatists.

No tanks were at the site on May 30, according to NATO. An image from June 6 shows the departure of the Russian military unit but the arrival of 8 main battle tanks. On June 11, 10 main battle tanks can be seen at the site, NATO said. Of these, three have been loaded onto the kind of low trucks "normally used to move tanks, likely indicating imminent movement by road."

NATO also cites video posted online showing tanks on the move in Makiivka and Snizhne in eastern Ukraine, in one instance accompanied by a vehicle flying a Russian flag.

"The tanks do not bear markings or camouflage paint like those used by the Ukrainian military. In fact, they do not have markings at all, which is reminiscent of tactics used by Russian elements that were involved in destabilizing Crimea," the NATO statement said.

It also accused Russian officials of having been "repeatedly misleading and evasive regarding their roles in both Crimea and eastern Ukraine."

If the latest reports are confirmed, the statement added, "this would mark a grave escalation of the crisis in eastern Ukraine in violation of Russia's Geneva commitments."

U.S. and EU leaders have warned that additional economic sanctions could be imposed on Russia if it further escalates its incursion into Ukraine.

Russian leaders in turn say Ukraine has failed to move to implement the framework for peace worked out in Geneva, Switzerland, in April. They say Ukrainian leaders have carried out a campaign of violence against people living in the largely pro-Russian east.

Mariupol operation

On Friday, the Kiev government announced that an operation targeting pro-Russian separatists in the city of southeastern port city of Mariupol had been successful.

Anton Geraschenko, adviser to Avakov, said more than 30 "terrorists" had been detained and their base destroyed. Other separatists are hiding in homes and basements for safety, he said.

Four Ukrainian soldiers were injured in the operation, he said.

Residents of the city may in the past have blocked Ukraine's security forces, he said, but "today, not one resident of Mariupol protected terrorists."

According to the Interior Ministry, the yellow-and-blue Ukrainian flag is flying once again over the main municipal building in Mariupol. The city has changed hands several times in the course of hostilities over the past several months.

But the unrest hasn't ended.

Pro-Russian separatists wielding automatic weapons and grenades attacked Ukraine border service vehicles Saturday near the city, Ukraine's State Border Service said. Five people were killed and seven others were wounded, the service said. The self-proclaimed Republic of Lugansk took credit for the attack, service spokesman Oleg Slobodyan said Saturday.

Diplomatic moves

Western leaders have accused Moscow of fomenting instability in eastern Ukraine and have urged Moscow to engage with the new leadership in Kiev.

On Thursday, Poroshenko and Putin "held a substantial and long phone conversation," the Ukrainian President's media office said. The leaders discussed Poroshenko's peace plan to resolve the situation in the east of Ukraine, it said.

Poroshenko has called on the rebels to lay down their arms and engage in talks.

Also Thursday, Vitaly Churkin, Russia's U.N. ambassador, said he would introduce a resolution on Ukraine at the U.N. Security Council in light of what he said was a deteriorating situation in the country.