It was a bitter setback for the Ukrainian forces, which have struggled to suppress an armed insurgency by foes of the new government. And it came only a week after Ukraine's new president, billionaire candy magnate Petro Poroshenko, spoke about a peace plan in his inaugural address.
Yet the deadliest single incident in the four-month-old conflict suggested the two sides were still far apart in their demands and talk of de-escalating the conflict remained premature.
The loss of the plane "will refocus attention on the fact that Russia does not seem to be doing very much to moderate the insurgency (or) the cross-border resupply of separatists," said Timothy Ash, an analyst at Standard Bank PLC.
The United States, meanwhile, rejected Russia's statements that it was not arming the separatists, saying Russia clearly had sent tanks and rocket launchers to the rebels, making sure the unmarked tanks were of a type not currently being used by Russian forces.
Nine crew and 40 troops were aboard the Il-76 when it went down early Saturday as it approached the airport at Luhansk, the Ukrainian prosecutor general's office said. The Russian-built Il-76 is a four-engine jet used to transport heavy gear and people.
Luhansk is in eastern Ukraine near the border with Russia, an area where separatists have seized government buildings and declared independence after holding disputed referendums. Ukrainian forces still control the Luhansk airport, however.
Defense Ministry spokesman Bohdan Senyk said the rebels used anti-aircraft guns and a heavy machine gun to down the plane, while the prosecutor general's office mentioned an anti-aircraft missile.
The plane's tail section lay with other pieces of scorched wreckage in a field near the village of Novohannivka, 20 kilometers (12 miles) south of Luhansk. An AP reporter saw a dozen or more armed separatists inspecting the crash site.
The death toll Saturday exceeded the 46 who died after a fire and shootings in Odessa on May 2 and the 12 troops who died May 29 when rebels shot down a helicopter near the eastern city of Slovyansk.
The Kiev government has accused Russia of permitting three tanks to cross the border into eastern Ukraine, where they were used by rebels. Russia denies supplying the separatists.
In Washington, the U.S. State Department said Russia had stockpiled both tanks and weapons for the rebels at a depot in southwest Russia.
"Separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers," U.S. State Department spokeswoman Marie Harf said in a statement. "Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area. We are confident that these tanks came from Russia."
NATO released images on Saturday that it said showed recent Russian tank movements near the border.
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."
Tensions between Ukraine and Russia escalated in February after pro-Russian President Viktor Yanukovych was driven from office by protesters who wanted closer ties with the European Union and an end to the country's endemic corruption.
Russia then seized and annexed Ukraine's Black Sea peninsula of Crimea after a disputed referendum. The U.S. and Europe rejected the annexation and responded with financial sanctions targeting individual officials they deemed to have played a role. They have held off on widening the sanctions to the Russian economy but have not ruled that out.
"Comments from U.S. officials are now quite specific, and I would expect the focus to return to sanctions next week," said Ash, the analyst.
Poroshenko met with Russian President Vladimir Putin at D-Day anniversary ceremonies in France and there were reports Russia might take steps to tighten control over its border. Russia says Russian citizens fighting with the Ukrainian separatists are volunteers who went on their own.
Before Saturday's incident, the Ukrainian health ministry said at least 270 people had died in clashes between government forces and armed separatists.
McHugh contributed from Kiev.