Ukrainian Prime Minister Arseny Yatsenyuk has said Russia wanted to start a third World War by occupying Ukraine "militarily and politically". He also warned of a conflict that could spread to the rest of Europe.

"Attempts at military conflict in Ukraine will lead to a military conflict in Europe," Mr Yatsenyuk told the interim cabinet in remarks broadcast live.

"The world has not yet forgotten World War II, but Russia already wants to start World War III."

An aide to interim president Oleksander Turchynov said any incursion by Russian forces across the border will be regarded as an invasion and the attackers will be killed.
             
"We will consider any crossing of the Ukrainian border by Russian troops into the territory of Donetsk or any other region as a military invasion and we will destroy the attackers," Mr Turchynov's chief-of-staff Serhiy Pashynsky was quoted as saying by Interfax-Ukraine news agency.

"We do not accept false declarations about humanitarian action," he said. 

A Ukrainian military helicopter exploded at a base near the eastern town of Kramatorsk after being hit by a rocket-propelled grenade, officials in Kiev said.

The Mi-8 helicopter was struck as it was on the tarmac of the base's landing zone. Its pilot managed to escape but was wounded, they said.

The nearby town of Kramatorsk is one of several under the control of pro-Kremlin gunmen.

Meanwhile, Ukrainian special forces have begun the second phase of their operation in the east of the country by mounting a full blockade of Slaviansk, an official on the presidential staff said.