NATO agreed Thursday to dramatically boost its defences with six command centres in eastern Europe and a spearhead force of 5,000 troops, to counter what the alliance called Russian aggression in Ukraine.

Secretary General Jens Stoltenberg insisted that the increase approved by defence ministers in Brussels was purely defensive, but it is likely to rile a Moscow that is more wary than ever of Western military intentions.

"This is the biggest reinforcement of our collective defence since the end of the Cold War," Stoltenberg told the ministers.

He said the decision to step up the 28-nation group's defences to the east were "as a response to the aggressive actions we have seen from Russia, violating international law and annexing Crimea."

France, Germany, Italy, Spain, Poland and Britain agreed to take the lead in forming the spearhead rapid reaction force, which would be available to deploy within a week in a crisis, Stoltenberg said.

The six "command and control" centres that will help the deployment of the force will be in Estonia, Latvia, Lithuania, Poland, Romania and Bulgaria, with a corps headquarters in Szczecin, Poland, he added.

All six countries were formerly in the orbit of the Soviet Union and have voiced deep concern about Russia's actions in Ukraine.

NATO is also set to boost its wider response force -- which would take weeks or months to deploy in a crisis -- from 13,000 to 30,000 troops.

Moscow has long been wary of NATO's relations with its former allies, especially in the case of Kiev's new pro-Western president Petro Poroshenko, who has said he wants Ukraine to join the alliance.

Russian foreign ministry spokesman Alexander Lukashevich said earlier Thursday that Moscow would "fully consider such decisions in our military planning" after reviewing the details.

He said Moscow was "deeply concerned" about the plan because "it is nothing else but a buildup of military capacities of the alliance" near Russia's borders.

- To arm, or not to arm? -

NATO accuses Russia of sending troops and equipment to support separatist rebels in eastern Ukraine who have been fighting troops loyal to the western-backed government since April, with the loss of nearly 5,400 lives.

The Kremlin denies giving any backing to the rebels.

Ukraine has recently urged the West to send weapons to help Kiev fight the rebels, but former Norwegian prime minister Stoltenberg refused to say whether he thought it was a good idea.

He said NATO itself, an alliance set up after World War II to counter a growing Soviet threat, does not officially have weapons and "that has to be up to each individual ally to decide" whether to arm Ukraine.

Stoltenberg however welcomed the news that French President Francois Hollande and German Chancellor Angela Merkel were heading to Kiev and Moscow to present a new peace plan.

US Secretary of State John Kerry was in Kiev for talks with Ukraine's leaders who are asking for "defensive" weapons.

Asked about reports that Washington was considering sending weapons to Kiev, US Defence Secretary Chuck Hagel would only say that it was "reviewing" its assistance.

Stoltenberg said the NATO measures were also motivated by new threats to the 28-nation military alliance from Islamist militants in the Middle East and North Africa, who are fuelling violence within Europe.

NATO aims to have the new 5,000-member spearhead force operational by 2016, officials said. It will involve member nations having troops ready to deploy within a few days in case of a crisis, with them taking it in turns to lead the force for one year.

The command and control centres are being set up to make it easier to deploy them.

In a further blow to Moscow, NATO will set up a joint training centre in the former Soviet state of Georgia, with which Russia fought a war in 2008.