Reports suggest Ukrainian government forces are poised to surround pro-Russian separatist rebels in Donetsk region as fierce fighting continues.
The rebels' commander is quoted as saying his forces have been cut off after Krasnyi Luch, a strategic town to the east, was captured by the army.
However, a rebel politician in Donetsk insisted there were still corridors from the city to the east.
The Ukrainian government says 13 soldiers were killed on Friday.
A civilian was killed in Donetsk today when a shell hit a street in the city's Kirovskyy district, the city's official authorities reported. Shells also hit the Petrovskyy district, they added.
The new government is battling an insurrection by pro-Russian separatist rebels in a conflict that is believed to have cost at least 1,500 lives since April.
In other developments
· The Russian navy "expelled" a US submarine from its territorial waters in the Barents Sea on Thursday, a navy source told Russian media
· Russia announced it was freeing five Ukrainian officers for "humanitarian reasons", a day after announcing their arrest for suspected war crimes in Ukraine
· The UK and US governments warned Russia not to use humanitarian assistance as a pretext for sending troops into the east
Igor Girkin, the rebels' Russian commander who has taken the nom-de-guerre Strelkov, was quoted by Russian media as saying that Krasnyi Luch had been "captured" after Cossacks defending the town "ran away".
A small detachment of rebels, he said, was still holding out in the town, which connects the city of Donetsk with Ukraine's Luhansk region on the Russian border.
"Novorossia's Donetsk-Horlivka group of troops is completely encircled," Strelkov reportedly added, referring to the area between Donetsk and the embattled town of Horlivka, 43km to the north-east.
Ukrainian security spokesman Andriy Lysenko said he could not confirm that government forces had taken Krasnyi Luch.
Commenting on the news from Krasnyi Luch, Donetsk rebel official Andrei Purgin told Russia's Itar-Tass news agency there were still two corridors out of the city of one million people - through the town of Makiivka and the local power station.
Meanwhile, in the city of Luhansk, second only to Donetsk in its importance to the rebels, there are fears of a humanitarian disaster.
The city council reported on its website (in Russian) today that the city of 425,000 people had been without electricity and power for a week and mobile phones and land lines were not working.
Parts of the city were still being bombarded and most shops were shut, although bakeries and some chemist's shops remained opened, it added, quoting residents.