Paris: North Atlantic Treaty Organisation (Nato) chief on Tuesday warned Russia against intervening further in Ukraine, urging Moscow to “step back” after pro-Kremlin militants seized government buildings in several cities in the east.

“Russia’s illegal aggression against Ukraine is the greatest challenge to Europe’s security in a generation,” Anders Fogh Rasmussen told reporters.

“I urge Russia to step back and not escalate the situation in east Ukraine,” he said in Paris, where he was attending a seminar on Nato reforms.

“If Russia were to intervene further in Ukraine it would be a historic mistake. It would have grave consequences for our relationship with Russia and it would further isolate Russia internationally,” Rasmussen said.

In a speech at the seminar, Rasmussen specifically called on Russia to pull back its troops from the border with Ukraine.

“Any further move into eastern Ukraine would represent a serious escalation, rather than the de-escalation that we all seek,” he said.

“We call on Russia to pull back the tens of thousands of troops it has massed on Ukraine’s borders, engage in a genuine dialogue with the Ukrainian authorities, and respect its international commitments.”

Pro-Russia activists have seized state buildings in the eastern cities of Kharkiv, Lugansk and Donetsk, where they have vowed to vote on joining Russia in an eerie echo of last month’s situation in Crimea, a Black Sea peninsula that has since been annexed by Moscow.

Kiev has accused Russia of fomenting the unrest and Washington warned the Kremlin to stop efforts to “destabilise Ukraine,” accusations that Moscow brushed off.

Russia instead warned Kiev on Tuesday against any use of force in Ukraine’s east, which it said could tip the country into civil war.

Rasmussen said Nato was considering how to adjust its defence planning in response to the crisis, but would not go into specifics.

“The evolving security situation in Ukraine makes it necessary to review our defence plans and look into how we could further strengthen collective defence,” he said.

“We are now in the process of considering such initiatives.”