Pro-Russian rebels have shot down a military transport plane in Luhansk in the bloodiest single day of fighting in eastern Ukraine since the conflict began in April, setting back Kiev's efforts to end the crisis and establish control of the region.

The Ukrainian government had appeared to finally be making progress this week after new president Petro Poroshenko spoke with Russian president Vladimir Putin on Thursday and Kiev's forces retook the port city of Mariupol on Friday.

Rebels shot down an Il-76 transport plane in Luhansk, killing all 40 paratroopers and nine crew members who were aboard. Surveillance camera footage from the city showed a mid-air explosion after an object – apparently a rocket – streaked into the sky, followed by a larger explosion and fire on the ground. A Kiev military analyst later reported that the empty tubes of two Igla handheld surface-to-air missiles had been found near Luhansk airport, the only significant piece of infrastructure Ukrainian forces have been able to retain in the rebel-held city.

Vladimir Inogorodsky, spokesman of the self-declared Luhansk People's Republic, confirmed rebels had shot the plane down with Igla missiles and said they were currently blockading the airport. Rebels declared they would not allow any more flights into the airport last week.

"No more airplanes have landed at the airport since last night, and if they do we will shoot them down," he said.

In the city of Krasnodon in the Luhansk region, 20 armed rebels seized the security service building overnight, according to a regional interior ministry spokeswoman.

Fighting also picked up in the neighbouring Donetsk region. Rebels killed three border guards and wounded four when they ambushed an armoured column in Mariupol with machine guns and rocket-propelled grenades on Saturday morning, the Interfax Ukraine news agency reported. On Friday, government forces had captured 30 rebels and weapons caches, and raised a Ukrainian flag over the rebel headquarters after a six-hour operation to take Mariupol back.

Rebels in the city of Gorlovka, which is held by former Russian lieutenant colonel Igor "Demon" Bezler, said they had shot down a government Su-25 fighter jet after a night air strike on the rebel headquarters that reportedly killed one civilian and wounded six rebels.

A local named Alexei told the Observer he had seen a missile fired from the ground but could not tell if it had hit the plane. The Luhansk and Donetsk region anti-terrorist operation spokesman Vladislav Seleznyov later denied that the jet had been downed.

After the heavy casualties on Saturday, the total number of servicemen killed in eastern Ukraine rose to at least 115. The health ministry said earlier this week that a total of 270 people had been killed since the start of Kiev's "anti-terrorist operation" in the east, most of them in the besieged rebel-held city of Slavyansk.

The operation began in April after pro-Russian protesters began seizing government buildings across eastern Ukraine, holding makeshift referendums to declare their independence and calling on Russia to deploy troops to the region.

In response to the downing of the transport plane in Lugansk, Poroshenko declared a day of mourning on Sunday and called a meeting of the national security and defence council. "All who took part in this large-scale, cynical terrorist attack will definitely be punished," he said in a statement. "Ukraine needs peace, but terrorists will receive an adequate response."

The latest attacks come amid still more allegations that Russia is supporting the rebels in the east with men and arms. Although Igla missiles such as those used in Lugansk could have been obtained when rebels seized government arms caches, they require special training to fire, New York University professor and Russian security services expert Mark Galeotti told the Observer.

On Thursday, three T-64 tanks reportedly entered from Russia at a rebel-controlled border crossing, and the Observer saw a convoy including three such tanks flying a Russian flag on a highway north of Donetsk late that evening. In his phone conversation with Putin on Thursday about ending the Ukraine crisis, Poroshenko reportedly told the Russian president it was unacceptable that these tanks had crossed the border, and the US state department later also condemned this development.

Nato Allied Command Operations published satellite images on Saturday that it said showed three tanks on transport trucks ready to be moved near the Russian city of Rostov-on-Don on 11 June. "If these latest reports are confirmed, this would mark a grave escalation of the crisis in eastern Ukraine in violation of Russia's Geneva commitments," it said in a statement.

Foreign secretary William Hague said on Friday that the "international community stands ready to impose further sanctions if Moscow continues to provoke instability and does nothing to stop further violence."