"We're going to work with our coalition partners to keep the coalition as strong as we can but we certainly understand that adjustments get made along the way", said Douglas Feith, US undersecretary of defense for policy, during a visit to NATO's headquarters.

"It is understandable that these countries are under pressure to adjust", he told journalists, acknowledging "substantial contributions and sacrifices" by coalition members.

Feith, who is to step down from his position by summer, was commenting on Warsaw's announcement on Tuesday that it would pull its troops out of Iraq at the end of the year, when the mandate of the UN multinational stabilization force in the country expires.

He said that coalition troop levels in Iraq were linked to the development of security forces in the volatile country, the intensity of the armed insurrection and the size of future international contributions.

The American official met Wednesday with ambassadors from the North Atlantic Treaty Organization's 26 members, whom he briefed about an update to US national defense and military strategy as well as a "quadrennial defence review".

One of the key ideas of the strategy, he said, was that the United States believed that "that a lot of national security problems that we face are of the nature that we cannot deal with them entirely with military means and we cannot deal with them by ourselves".