Ukraine's government forces and pro-Russian separatists have accused one another of launching a major new attack in the country's east.

The rebel forces began a "large-scale offensive against Ukrainian positions" close to their stronghold of Donetsk overnight on Wednesday, using about a dozen tanks and up to a thousand troops, the Ukrainian army general staff said in a statement.

It said government troops had fought back against the attack using heavy artillery.

Fighting has intensified in recent weeks around key hotspots on the frontline in eastern Ukraine despite a truce brokered by the West in February.

Vladimir Kononov, the chief of the separatist forces, told Russia's Interfax news agency that at least 15 people, including civilians, were killed around Donetsk.

"The Ukrainian side has carried out a provocation and started shelling our positions all along the front," Kononov was quoted as saying.

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Mariinka and Georgiivka, areas under the control of Ukrainian forces about 30km from Donetsk, were under "intense shooting" since the morning, Vyacheslav Abroskin, chief of the Donetsk region's police force, which is loyal to Kiev, told the AFP news agency.

"The enemy is shooting on Mariinka using Grad [rockets] as well as tanks," he said, adding that there were victims but did not give figures.

He said the checkpoints at Georgiivka and Kurakhove, about 40km from Donetsk, were closed to traffic owing to the shooting.

According to Interfax, a hospital in Donetsk suffered a blackout and about 90 wounded people had to be evacuated.

In another related development, more than 900 miners were trapped underground after Wednesday's clashes caused a power failure at two large mines, Skochinsky and Zasyadko, in Donetsk.

According to the territory's emergency services, all miners were evacuated from Zasyadko mine by the evening, but the rescue operation was still going on for more than 300 miners trapped in Skochinsky.

The conflict between the Ukrainian army and separatist forces has killed more than 6,400 people since April 2014.

Ukraine and the West accuse the Kremlin of supporting and arming the pro-Russian separatists, which Moscow denies.