Speaking ahead of a meeting of NATO defense ministers in Brussels on Thursday, NATO Secretary-General Jens Stoltenberg said the representatives were expected to agree to the measures, which he insisted were purely defensive.

"This is something we do as a response to the aggressive actions we have seen from Russia, violating international law and annexing Crimea," he told reporters.

The strengthened rapid response force was set to include a "spearhead" force of 5,000 troops able to deploy anywhere within a few days, which was announced at a previous NATO conference last September and for which details were expected to be agreed on Thursday.

Other measures included setting up six regional command and control centers in eastern European nations, namely Estonia, Latvia, Lithuania, Poland, Romania and Bulgaria, to ensure the new forces could be mobilized quickly.

A multinational headquarters for the alliance in Szczecin, Poland, would also be expanded. The military boost was also motivated by threats from extreme Islamist militants in the Middle East and North Africa.

"Our decisions make clear that NATO is determined to defend all allies against any threat, from any direction," Stoltenberg said.

'Crisis deepening'

When asked if the measures would fuel a Cold War-style escalation with Moscow, he said NATO was acting in defense because of Russia's actions.

"In Ukraine, violence is getting worse and the crisis is deepening," he said, adding: "Russia continues to disregard international rules and to support the separatists with advanced weapons, training and forces."

Russia has denied sending troops and weapons to support the separatists fighting Kyiv's armed forces in eastern Ukraine.

German Defense Minister Ursula von der Leyen described the expected increase in troops as a "sign of closeness and determination" on the part of NATO.

It comes at a time when the US is considering sending weapons to help Ukraine's army. US Secretary of State John Kerry is making a visit to Kyiv on Thursday. German Chancellor Angela Merkel and French President Francois Hollande also announced that they would travel to Kyiv on Thursday to present a new proposal to end the conflict.

Stoltenberg is due to meet with Russian Foreign Minister Sergey Lavrov at the Munich Security Conference this weekend.

According to UN figures, the conflict between separatists and Kyiv's forces in eastern Ukraine has claimed more than 5,300 lives since it broke out in April 2014.

se/sms (AP, AFP, Reuters, dpa)