GermanChancellor Angela Merkel has demanded an explanation from Russia's PresidentVladimir Putin amid reports that Russian troops have launched an incursion intosouth-east Ukraine.
Ukrainesaid Russian forces had crossed the border and were supporting separatistattacks.
The USState Department said it suspected a Russian-directed counter-offensive wasunder way.
Russia hasrepeatedly denied arming or covertly supporting the rebels.
The BBC'sBarbara Plett Usher in Washington says the suspicion is that Moscow is openinga new front to divert Ukrainian forces from the besieged cities of Donetsk andLuhansk, where they have made significant advances against pro-Russianseparatists.
However,Denis Pushilin, a rebel leader in Donetsk, told a news conference that Russiawas not involved in the current offensive.
"IfRussia entered into the war the counter-offensive would already be in Kiev. Fornow, we do without outside help," he said.
He addedthat the rebel forces were receiving more volunteers, some from as far away asSerbia.
Earlier onWednesday, the mayor of the Ukrainian port of Novoazovsk on the Sea of Azovsaid rebel tanks and armoured vehicles had entered the town.
Rebels havebeen trying for weeks to break out of an area further north in the Donetskregion where they are almost encircled.
(BBC News)
Bakudaily.Az
