LUHANSK, Ukraine — Pro-Russia separatists in Ukraine used a shoulder-fired rocket to shoot down a Ukrainian military transport jet as it was trying to land on Saturday, killing all 49 people on board, the military said.
The attack was the deadliest episode for the military since the unrest began in eastern Ukraine.
Ukrainian President Petro Poroshenko vowed to strike back against the separatists and announced a national day of mourning.
The plane was shot down at the airport in Luhansk, 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.
Ukraine claims that Russia has been arming the separatists, who have carved out enclaves they want to see become part of Russia.
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.
Last week, Ukraine said three tanks crossed the border into eastern Ukraine, and separatists have acknowledged having possession of them.
Russia has denied involvement with the tanks, but the U.S. State Department and NATO have said they have proof that heavy military equipment has come across the border in recent days.
Mr. 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,” Mr. 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 Engel (D., N.Y.), a member of the House Foreign Affairs Committee, and Sen. Bob Corker (R., Tenn.) of the Senate Foreign Relations Committee, urged the Obama Administration to impose tougher sanctions on Moscow, in view of recent developments.
Secretary of State John 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 weapons, according to a State Department official.
In Kiev, meanwhile, angry Ukrainians showed up at the Russian Embassy on Saturday to protest the attack on the plane.
State Department spokesman Jen Psaki 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, Mr. Poroshenko’s security detail said they found a crudely made bomb outside the presidential office, near the entrance, and defused it.
In the east, there were reports of intensified fighting between government troops and the pro-Russia 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.
In the city of Mariupol, which the Ukrainian army 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.