Pro-Russian rebels have opened up a new front in south-eastern Ukraine.
Reports say they have taken the town of Novoazovsk, on the Sea of Azov, and are heading for the key port of Mariupol, reports GHN based on BBC.
Ukraine says Russian forces have crossed the border and are supporting the rebel attack, but the Moscow government has repeatedly denied arming or covertly supporting the rebels.
There are suspicions that Russia wants to divert Ukrainian forces from the besieged cities of Donetsk and Luhansk.
Government forces have made significant advances against the separatists in recent weeks.
German Chancellor Angela Merkel has demanded an explanation from Russia's President Vladimir Putin amid the reports of an incursion.
The US said it suspected a Russian-directed counter-offensive was under way.
Ukrainian forces 'leaving'
Reports from journalists and military on the ground suggest that Novoazovsk had been captured by the rebels.
"The guys from the east shot the positions of the Ukrainian army and the army left Novoazovsk," freelance journalist Petr Shelomovsky told the BBC.
"Since the morning, they've been leaving the town and we've probably seen the last armoured personnel carrier leaving the place."