Ukrainian President Petro Poroshenko vowed to strike back against pro-Russian separatists who shot down a military transport plane early Saturday morning, killing all 49 people aboard and dashing hopes of defusing the conflict anytime soon.

Poroshenko announced a national day of mourning after the plane was shot from the sky as it was landing at the airport in Luhansk, an eastern city where separatists have taken over government buildings and declared an independent state. The Ukrainian military holds the city’s airport but little else.

According to the Ukrainian military, rebels used anti-aircraft weapons and large-caliber machine guns against the Ilyushin-76 transport plane, which carried 40 paratroopers flying in as part of a routine rotation and a crew of nine. There were no survivors.

Videos filmed after the plane crashed about 1:10 a.m. showed flames lighting up the nighttime horizon. By daylight, debris could be seen strewn over a large area outside the city.

Russian officials did not comment directly on the attack, but they called for an immediate cessation of Ukrainian military operations in the east, to be followed by a dialogue between representatives of the region and the Kiev government.

1 of 22 Full Screen Autoplay Close June 13, 2014 Skip Ad × Ukraine retakes Mariupol, part of border View Photos Ukrainian forces ousted pro-Russian separatists from a port city in the east and regained control of a stretch of the border with Russia on Friday, the government in Kiev announced. The attack on separatist strongholds in Mariupol, the second-largest city in the Donetsk region, was over in less than six hours, an Interior official said. Caption Ukrainian forces ousted pro-Russian separatists from a port city in the east and regained control of a stretch of the border with Russia on Friday, the government in Kiev announced. The attack on separatist strongholds in Mariupol, the second-largest city in the Donetsk region, was over in less than six hours, an Interior official said. June 14, 2014 Pro-Russian fighters secure the area around a downed Ukrainian military transport plane in the eastern city of Luhansk, Ukraine. The Ukrainian prosecutor's office has confirmed that 49 people were killed after separatists shot down the Ilyushin-76 air force plane as it was approaching Luhansk’s airport. Mstyslav Chernov/European Pressphot Agency Buy Photo Wait 1 second to continue.

The deadly attack came as tensions between Moscow and Kiev are rubbing raw. Ukraine claims that Russia has been arming separatists, who have carved out enclaves they want to see become part of Russia. Last week, Ukraine said three tanks crossed the border into eastern Ukraine, and separatists have acknowledged having possession of them. Although Russia has denied involvement, the U.S. State Department and NATO have said they have proof that heavy military equipment, including tanks and rocket launchers, has come across the border in recent days.

Poroshenko called a meeting of his national security and defense advisers after the plane was downed and said Ukraine would simultaneously work for peaceful negotiations and go after the perpetrators.

“Ukraine needs peace,” he said. “But terrorists will receive an adequate response.”

In Moscow, Foreign Minister Sergei Lavrov said Saturday that negotiations would be impossible as long as the Ukrainian military continues its activity in the east.

“Rather than extending a hand to these people, inviting them to the negotiating table and agreeing how to continue to live in the country all together, the military operation continues,” Lavrov said on a Russian television program. “Naturally, under the roar of cannon fire, shelling and strikes from combat aircraft, such dialogue is impossible.”

The Russian Foreign Ministry also complained about what it said were several border violations, declaring that if they continue, Russia would respond.

“If the Ukrainian side continues to violate the state border, all necessary measures will be taken to stop them,” the ministry’s statement said, citing two airspace violations since June 5, as well as a ground violation Friday in which an armored vehicle drove about 500 feet beyond the border.

The claims were impossible to verify, but they appeared to be laying the groundwork for a military escalation along the border.

NATO released satellite images Saturday that it said supported the assertion that Russian tanks had crossed the border into Ukraine on Thursday, although the images did not definitively establish that the tanks had come from Russia.

In the images, 10 tanks are seen Wednesday at a staging area near Rostov-on-Don on the Russian side of the border, NATO said. Three of the tanks are on transport trucks of the sort usually used to move tanks by road, NATO said.

In Washington, Rep. Eliot L. Engel (D-N.Y.), a member of the House Foreign Affairs Committee, and Sen. Bob Corker (Tenn.), ranking Republican on the Senate Foreign Relations Committee, separately urged the administration to impose tougher sanctions on Moscow, in view of recent developments.

Secretary of State John F. Kerry told Ukrainian Prime Minister Arseniy Yatsenyuk in a phone call Saturday that the United States is willing to “raise the costs” for Russia if it does not stanch the flow of arms, according to a senior State Department official.

In Kiev, meanwhile, angry Ukrainians showed up at the Russian Embassy on Saturday afternoon to protest the attack on the plane. Shouting that they were there “for our military,” they pelted the building with eggs and upturned the vehicles of diplomats and staff on the sidewalk in front.

Russia on Sunday condemned the attack against the embassy, saying the Ukrainian government is obligated to protect its diplomatic outpost in the capital city.

“The Ukrainian authorities’ failure to guarantee the Russian embassy’s security is absolutely unacceptable,” Kremlin spokesman Dmitry Peskov told the Interfax newswire agency.

State Department spokeswoman Jen Psaki also condemned the attack on the embassy and urged Ukraine to abide by its obligations to provide adequate security for diplomats.

The commander of a militia group made up of volunteers fighting the separatists in the east, took to Facebook vowing to avenge the downing of the plane.

“We won’t forget,” wrote Semyon Semenchenko. “We won't forgive."

Further escalating tensions, Poroshenko’s security detail said they found a crudely made bomb outside the presidential office, near the entrance where cars drive through, and defused it before it exploded.

In the east, there were reports of intensified fighting between government troops and the pro-Russian separatists.

The Ukrainian air force carried out a strike on a police station occupied by separatists in the city of Horlivka, in the Donetsk region. And in the city of Mariupol, which the Ukrainian army proudly announced it had retaken from rebels Friday, attackers fired a rocket-propelled grenade at a car carrying personnel from the national border service. Five agents died, the border service said.

Alex Ryabchyn in Kiev and Karen DeYoung in Washington contributed to this report.