By Adrian Croft and Robin Emmott
BRUSSELS (Reuters) - The head of NATO said on Wednesday the alliance would not be forced into a new arms race with Russia but said what he called Moscow's aggression in Ukraine had compelled it to strengthen its defenses.
The United States announced plans this week to station tanks and heavy weapons in NATO member states on Russia's border, shortly after President Vladimir Putin said Moscow would add 40 missiles to its nuclear arsenal.
"We will not be dragged into an arms race, but we must keep our countries safe," NATO Secretary-General Jens Stoltenberg told reporters at a meeting of alliance defense ministers.
A Russian official last week accused NATO of pushing Russia into an arms race by stepping up its military activity around its borders, not least in the formerly Soviet Baltic states.
Stoltenberg said the U.S. decision to store equipment in eastern Europe was a prudent response to Russia's actions.
He said the defense ministers had agreed to increase the strength of NATO's rapid response force, including its air, maritime and special operations components. The force will now consist of up to 40,000 personnel, up from 13,000 previously.
Within that force, NATO plans to create a 5,000-strong "spearhead" force, part of which could move within 48 hours.
The United States said on Monday it would contribute special operations forces, intelligence and other high-end military assets to the new "spearhead" force.
Canadian Defence Minister Jason Kenney announced on Wednesday his country was ready to offer air-to-air refueling, reconnaissance or transport planes for the force.
U.S. Defense Secretary Ash Carter, who is attending his first NATO ministerial meeting, said on Tuesday the Baltic states - Estonia, Latvia and Lithuania - as well as Bulgaria, Romania and Poland had agreed to host the U.S. arms and heavy equipment.
It was one of a range of steps Washington and NATO are taking to reinforce allies in eastern Europe after Moscow's annexation of Ukraine's Crimea region last year and what NATO says is Russia's military support for rebels in eastern Ukraine.
The defense ministers also agreed on Wednesday to give NATO's military commander new powers to prepare the rapid reaction force in a crisis, although governments would still have to give the go-ahead before it could go into action.
Polish Defence Minister Tomasz Siemoniak reiterated his country's call for NATO forces to be based permanently on its soil.
Siemoniak told Reuters he hoped regular rotations of alliance forces through Poland would gradually be transformed into a permanent presence.
Despite the alliance's concerns over a more assertive Russia as well as other security challenges, only five of its 28 member states are expected to meet NATO's target of spending at least two percent of their national output on defense this year.
(Additional reporting by Sabine Siebold and Phil Stewart; editing by Ralph Boulton and Gareth Jones)