At least 49 people were killed when pro-Russian separatists shot down a Ukrainian military transport plane at the eastern city of Lugansk, a military spokesman says.
"This is our preliminary information," Ukrainian defence spokesman Vladyslav Seleznyov told AFP following Saturday's attack.
"Details about how many people were on board are still being checked."
‘‘The terrorists cynically and treacherously fired with a large-calibre machinegun hitting an Ilyushin-76 of the Ukrainian air force which was carrying troops on rotation and was about to land at Lugansk airport,’’ the ministry said in a statement. The four-engine jet was also carrying ‘‘equipment’’.
The ministry offered its condolences to the families of the "killed soldiers".
Lugansk, which is close to the Russian border, is the main city of one of two eastern regions hit by pro-Moscow separatist violence and which has declared its independence from Kiev.
The United States has accused Russia of sending tanks and other heavy weapons to separatists in Ukraine, supporting accusations made by the Ukrainian government.
A convoy of three tanks, several multiple rocket launchers and other military vehicles crossed the border near the Ukrainian town of Snizhne, US officials said. The Ukrainian army reported on Friday that it had destroyed two of the three tanks and several other vehicles in the convoy.
"This is unacceptable," said Marie Harf, the deputy State Department spokeswoman. "A failure by Russia to de-escalate this situation will lead to additional costs."
US Secretary of State John Kerry complained earlier this week about the flow of Russian arms to separatists in Ukraine in a phone call to Sergey Lavrov, the Russian foreign minister.
A spokesman for Russian President Vladimir Putin, Dmitry Peskov, said a Ukrainian armoured personnel carrier had crossed into Russia for unclear reasons and was abandoned there.
At the meeting of the Group of 7 industrial powers last week, President Barack Obama warned Mr Putin that the West would impose "additional costs" on Russia if its provocations continued.
"Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area," the State Department said. "We are confident that these tanks came from Russia."
Even before the State Department's statement, Ukraine was having one of its better days in eastern Ukraine, with government forces winning control of the port of Mariupol, the second-largest city in the separatist region of Donetsk.
The Ukrainian assault in Mariupol left five pro-Russian militants dead and four Ukrainian soldiers wounded and ended with the hoisting of a Ukrainian flag at City Hall as the military routed the last separatists from the city's administration buildings.
The victory was as much a propaganda win as a military one, as pro-Ukrainian forces had controlled most of Mariupol for weeks..
In fighting near the besieged town of Slovyansk, a Grad ground-to-ground rocket of the sort said to be in the weapons convoy from Russia killed one person and wounded several others. Residents said the rocket went off course and hit a vegetable market in the village of Dobroyepole.
Over the long term, Ukraine, with its far larger though badly equipped and poorly trained army, has more forces, and it is unclear how long the separatists can hold out without more Russian support. That is something Moscow cannot offer openly without risking more severe Western sanctions.
The result is misdirection and sleight of hand, and a conflict of endless puzzles and mind games.
The daytime journey of the three tanks through eastern Ukraine, which was recorded in multiple videos and witnessed by Western reporters, could not have been more obvious, and yet the convoy was too small to serve a military purpose. Was it a warning?
A rebel leader, Denis Pushilin, told Russian state television on Friday that the separatists had tanks but that it was "improper to ask" where they had gotten them.
Russia ratcheted up the economic pressure on Ukraine on Friday in their dispute over natural gas supplies and prices. Gazprom's chief executive, Aleksei Miller, said that the company, a state-controlled energy giant, would not negotiate further with Ukraine over gas prices until Kiev paid its debts for past deliveries and would adhere to a Monday deadline demanding prepayment for future supplies.
AFP, New York Times