Violence flared in volatile eastern Ukraine ahead of the country's presidential elections, with Kiev calling for an urgent U.N. security council meeting Thursday after a deadly assault on troops by pro-Russian separatists in Donetsk.
At least 11 Ukrainian soldiers were killed by pro-Russian separatists who ambushed their checkpoint in the Donetsk region - the deadliest attack in weeks.
Ukraine's Prime Minister Arseniy Yatsenyuk said after the attack that Russia was intentionally trying to destabilize the elections and called for an urgent United Nations Security Council meeting.
Hours after the assault on the checkpoint, rebel leaders in neighboring Luhansk announced they were to impose martial law until government troops ceased their offensive.
Thursday's events underscore the deep divisions Ukraine still faces ahead of Sunday's presidential vote, an election separatists have pledged to derail.
At the scene of the ambush near the village of Blahodatne, journalists from The Associated Press said they saw the dead bodies of at least 11 government troops and three charred armored personnel carriers. About 30 other soldiers were injured, some of them seriously, and state helicopters had arrived to transport the wounded, the AP said.
A rebel commander in the nearby town of Horlivka claimed responsibility, telling the news agency: "We destroyed a checkpoint of the fascist Ukrainian army deployed on the land the Donetsk Republic."
The commander, who wore a balaclava and identified himself by his nom de guerre, "Bes," Russian for "demon," displayed heavy weaponry taken from the men that said were "trophies."
Pro-Russian separatists have occupied large areas of eastern Ukraine in opposition to the Western-backed Kiev government that was imposed after the ouster of the Moscow-friendly President Vikor Yanukovych.
Following a referendum earlier this month, the Donetsk and Luhansk regions set up so-called "people's republics," declaring their independence from Kiev and demanding to be annexed by Russia.
Hours after the ambush Thursday, members of the Luhansk People's Republic said their leader was to announce that the region would be under martial law until Ukraine withdrew its troops.
"Martial law will be in force starting from May 22 and until the Kyiv authorities withdraw their troops from the territory of the Luhansk People's Republic," spokesman Volodymyr Inohorodskykh told Interfax Ukraine.
Authorities in Kiev see Sunday's vote as a chance to defuse tensions and stabilize the country - but have acknowledged it might be difficult to stage the poll in some of the more volatile parts of the country.
The Associated Press contributed to this report.