By Nick Childs BBC Pentagon correspondent

The US Defence Secretary has been presenting a plan for a new Nato rapid reaction force to alliance defence ministers at an informal meeting in Poland, but it is a meeting overshadowed by Iraq.

The Bush administration has made it clear that it sees the crisis over Iraq as a test of the UN's credibility.

But many in the Bush administration, and particularly in the Pentagon, believe that Nato too is on trial, both over Iraq and because of events since the 11 September attacks on the United States.

Iraq was high on the agenda at Nato meetings

Washington's allies will want to know more about the rationale behind any possible attack, and what Washington may want from them.

But Pentagon insiders say that President Bush's nomination of America's most able senior officer, General James Jones, as the next Nato supreme commander in Europe, is evidence of the value Washington still places on the alliance.

Others point out that General Jones will be the first US Marine Corps officer to hold the post. That, they say, underlines the fact that Washington wants Nato to be more ready to operate outside its traditional areas.

Rifts in the alliance

Publicly, there was a closing of ranks after 11 September.

For the first time ever, Nato invoked the collective self-defence article of its founding treaty, and Nato Awacs aircraft were sent to help patrol US skies.

But few Nato countries played a major role in the early stages of the military campaign that the United States launched in Afghanistan, partly because few could.

Many in Washington questioned Nato's relevance after those attacks. Many Nato partners felt snubbed.

The recent strains over Baghdad, and the open row between Washington and Berlin, have not helped the atmosphere.

US presses for change

But, beyond that, Washington wants Nato to adapt more quickly to what it sees as the new security threats.

Washington is also worried that the capabilities gap between US and European forces is growing, and it wants its European allies to spend more on defending themselves.

The US wants Europe to have more "airlift capability", but it comes at a cost

That is the main thrust of the new US plan for an alliance rapid response force.

It would involve land, sea, and air forces able to deploy in as little as seven days. It would take two years to set up.

A wider reform of Nato's command structure is also under way.

Most alliance members agree that change is needed, and there has been a general welcome for the US plan.

European concerns

But there will be concerns in Europe about how much the US plan could cost.

Some of the developments that the Americans think the Europeans urgently need - like extra airlift capability - cost a lot of money.

The other big question for many is the direction in which Washington is pushing, in terms of how such forces would be employed in the future.

The new US security strategy, unveiled last week, emphasised the possibility of pre-emptive action, and that causes unease among some Nato members.

But some supporters of Nato among the US military say that it is important for the two sides of the Atlantic to engage in a dialogue over how force might be applied in the future, rather than adopt entrenched positions.