The II-76 cargo plane, carrying equipment and food, as well as personnel, crashed early Saturday. The plane's tail section lay with other pieces of scorched wreckage in a field near the village of Novohannivka, 12 miles south of Luhansk, The Associated Press reported. An AP reporter saw a dozen or more armed separatists inspecting the crash site.
The rebels "cynically and treacherously" downed the plane, the Ukraine Defense Ministry statement said.
It is the deadliest single incident since the protest movement began four months ago. On May 2, 46 people died after a fire and shootings in Odessa.
Russian separatists in Lugansk declared independence from Ukraine on May 12, after holding a disputed referendum. The area in the eastern Ukraine borders Russia and Donetsk, also an epicenter of Russian separatism. The protest movement began in February when Ukrainians who sought closer ties with the European Union drove pro-Russian Ukrainian President Viktor Yanukovych from office.
The U.S. and Ukraine have accused Russia of arming the separatists with tanks and rocket launchers. Russia denies it sent weapons and tanks over the border.
Ukraine's new president, billionaire candy magnate Petro Poroshenko, on Saturday called an emergency meeting of Ukraine's national security council and declared Sunday a day of national mourning, AP reported. Afterward, the president scolded the head of the country's SBU security service, referring to "omissions" in measures to protect military aircraft from attack. He called for "a detailed analysis of the reasons" and hinted that personnel changes were imminent.
Poroshenko said last week in his inaugural address that he would seek a peaceful resolution to end the bloodshed. He said he would seek a cease-fire that includes amnesty for separatists to lay down their arms, safe conduct for Russian fighters back to Russia and broad decentralization of authority to Ukraine's region.
"We are highly concerned by new Russian efforts to support the separatists," State Department spokeswoman Marie Harf said Friday.
A convoy of three T-64 tanks, several BM-21 or Grad multiple rocket launchers and other military vehicles crossed from Russia into Ukraine near the Ukrainian town of Snizhne, Harf said. She said Secretary of State John Kerry expressed his concern to Russian Foreign Minister Sergei Lavrov on Wednesday and again on Saturday.
NATO released images on Saturday that it said showed recent Russian tank movements near the border, the AP reported.
The tanks seen in Ukraine, NATO said, "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."
Senior Russian and Ukrainian officials met Saturday in the Ukrainian capital to settle a dispute over payments for natural gas. As the talks got underway, protesters hurled eggs and overturned diplomatic cards outside the Russian Embassy.
The U.S. condemned the attack pn Saturday and urged Ukrainian authorities to provide adequate security to the embassy.