German Chancellor Angela Merkel has demanded an explanation from President Putin over reports that Russia has sent soldiers to fight in the southeast of Ukraine.
"The latest reports of the presence of Russian soldiers on Ukrainian territory must be explained," said Merkel's spokesman Steffen Seibert. "She emphasised Russia's major responsibility for de-escalation and watching over its own frontiers."
The Ukrainian government has accused Moscow of supporting pro-Russian rebels who have opened a new front near the port of Mariupol, Reuters reports.
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The BBC's Barbara Plett Usher said that the suspicion in Washington was that Russia is leading an assault on the southeast of the country to divert Ukrainian forces from the besieged cities of Donetsk and Luhansk.
Denis Pushilin, a rebel leader in Donetsk, insisted that Russia was not involved in the fighting. "If Russia entered into the war, the counter-offensive would already be in Kiev," he said. "For now, we do without outside help."
The separatist resurgence "dimmed the glimmer of optimism" that had begun to emerge from talks held in Minsk between Putin and President Poroshenko, The Times says. The two leaders had previously been "cautiously positive" about their discussions and Poroshenko had promised to deliver a "road map" to peace.
Some analysts say that the latest fighting could be an attempt to establish a land corridor from Russia to Crimea, which would also offer control over the entire Sea of Azov.
Such a corridor would "transform the geography of Europe", The Times says.
Ukrainian military spokesman Andriy Lysenko said on Wednesday that a column of five armoured personnel carriers had been seen crossing the Ukrainian border.
"Five armoured infantry carriers and one Kamaz truck entered Amvrosiyivka with men in them," Lysenko told journalists in Kiev. "If this tactical group got lost and accidentally came into Ukraine like the paratroops of the 98th paratroop division, then it remains for us to remind them that they can return to Russia, taking an easterly direction," said Lysenko.
