"The Ukrainian military is conducting an active assault on regions under temporary control of Russian mercenaries," Lysenko told a news conference in Kiev.

In Donetsk local officials said artillery fire had damaged residential blocks, houses, power lines and a gas pipeline. The city, with a pre-war population of nearly 1 million, has largely become a ghost town since rebels dug in for a stand in the face of advancing Ukrainian troops.

The site of the crash of the Malaysian airliner has yet to be secured or thoroughly investigated, more than 10 days after the crash. After days in which bodies lay untended in the sun, rebels gathered the human remains and shipped the bodies out, and turned over the flight recorders to a Malaysian delegation.

But the wreckage itself is still largely unguarded, and much of it has been moved or dismantled in what the rebels say was part of the operation to recover the bodies. No full forensic sweep has been conducted to ensure all human remains have been collected. Both sides accuse the other of using fighting to prevent the investigation.

The Organization of Security and Cooperation in Europe said its experts attempting to reach the crash site with investigators Australia and the Netherlands were forced to return to Donetsk for "security reasons".

A rebel leader, Vladimir Antyufeyev, told reporters in Donetsk that separatist fighters escorting the international experts to the site encountered fighting and turned back.

Antyufeyev, who like most of the senior rebel leadership is an outsider from Russia, also blamed the "senseless" Ukrainian army for trying to destroy evidence at the crash site under cover of fighting. (rt/ez)