Secretary of State John Kerry spoke Saturday with Ukrainian Prime Minister Arseny Yatsenyuk after Russian separatists shot down a transport plane, killing dozens, according to the State Department.

They spoke by phone after the plane was shot down overnight in Ukraine's ongoing political upheaval.

The air strike killed 49 people, the flight crew and troops, and was the deadliest incident in Ukraine’s four-month-old conflict with the separatists – suggesting the two sides were still far apart in their demands and that talk of de-escalating the conflict remains premature.

In February, the country ousted Moscow-backed President Viktor Yanukovych amid allegations of corruption and held an election last month in which citizens selected pro-Western President Petro Poroshenko, a billionaire candymaker.

However, Russian President Vladimir Putin has taken control of Ukraine’s eastern peninsula of Crimea. And the pro-Russian separatists continue to cause deadly violence and try to take control of strategic locations throughout the country.

Kerry also expressed concern about the continued flow of heavy weaponry and militants entering Ukraine from the Russia border. And he reiterated U.S. support for Ukraine’s efforts to defend its border and for Poroshenko’s peace plan, the State Department said.

He also spoke with Russian Foreign Minister Sergey Lavrov on the same issues and pressed him to “make clear” Russia's commitment to de-escalating the violence and to peace by ending the flow of weapons and support to separatists.

The State Department said Kerry also discussed the escalating threat of violence posed by Islamic State of Iraq and the Levant, the Al Qaeda splinter group behind the recent uprisings in Iraq, and the need to remove Syria's remaining chemical weapons material as soon as possible.

The downed plane this weekend 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 Poroshenko spoke about a peace plan in his inaugural address.

The United States has already rejected earlier Russian statements that it was not arming the separatists, saying Moscow 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. However, Ukrainian forces still control the Luhansk airport.

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, 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 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," agency 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."

Poroshenko met with Putin earlier this week 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.

This story is based in part on wire service reports.