NATO on Wednesday warned pro-Russian separatists against grabbing more land in eastern Ukraine and stressed Moscow has a "special responsibility" to help restore peace as fighting escalates.

It said the members of the 28-nation, US-led alliance expressed "their serious concern about the recent sharp escalation of violence" when they met in Brussels.

"Russia has a special responsibility to find a political solution," NATO acting spokeswoman Carmen Romero said in a statement.

"Any attempt by the Russian-backed separatists to take over more of Ukraine's territory would be unacceptable to the international community," she said.

The rebels currently control parts of Ukraine's Lugansk and Donetsk regions and have threatend to expand their holdings further westwards.

The NATO allies stressed the need for all sides to "de-escalate tensions and exercise restraint" while calling for "full implementation" of the peace accords reached in Minsk in February.

It said monitors from Europe's OSCE mission must be able to "do their job safely and without restrictions" after the organisation said they faced "unprecedented" harassment during the spike in violence, mostly from the separatists.

"NATO stands firm in our support for Ukraine's sovereignty and territorial integrity," it said.

It renewed its support for "an independent, sovereign and stable Ukraine" that will pursue the path of democracy as the best way to ensure security in the Euro-Atlantic area.

It vowed to continuing following events closely.

Ukraine's government troops have been fighting pro-Russian separatists since April last year, which has claimed the lives of nearly 7,000 people.

While the conflict eased after the February truce agreement, fighting has intensified in recent days.

The Western-backed Kiev government and the insurgents on Monday reported the deaths of at lest 10 soldiers and civilians, the worst bloodshed in more than a month.

The fighting has stirred the highest tensions since the Cold War ended more than two decades ago as the West accuses Russia of not only arming the rebels but sending in troops to support them. Moscow denies the charges.