By Paul Reynolds BBC Washington correspondent

The post 11-September world has thrown the United States and Russia into a much closer alignment, but the exact terms of the new relationship have yet to be worked out - and have already produced tensions within the Bush administration.

There are those in Washington who feel that there is an opportunity here.

One senior Western diplomat, with experience of both Moscow and Washington, told the BBC that there was a chance of reaching the kind of closeness which Boris Yeltsin used to talk about, but never achieved.

Then there are others who want to be more cautious, on the grounds that President Putin cannot be wholly trusted.

US missile defence plans are a source of friction

Two issues have highlighted this division.

The first is a proposal to form a new Nato council which would bring Russia in as a full partner.

This council would not be 19 plus 1 but 20, signalling that Russia was a full player.

These kinds of numerical symbols are often important in international diplomacy.

The new council is strongly supported by many Nato members.

Britain's Tony Blair has been promoting it strongly, and France also believes that Russia should be rewarded for making what the French call a "strategic choice" to side with the West.

Plans for this council will be examined by Nato defence ministers - they will have to decide what powers it will have, what issues it will cover, and what exactly the Russian role will be.

Nato at 20

According to the New York Times, the "Nato at 20" plan, as it is known in the jargon, displeased the US Defence Secretary Donald Rumsfeld, who felt that it was all a bit hasty.

Mr Rumsfeld is a man who knows what he likes and does not like and is not afraid to speak his mind.

Donald Rumsfeld wants to proceed with caution

He reportedly tried to get a recent Nato ministerial meeting to tone down a reference to it, against the views of the Secretary of State Colin Powell.

At one stage, the American Nato Ambassador, Nicholas Burns, said that he could not address the meeting as he had conflicting instructions.

The Powell lined prevailed, but the commitment to Nato at 20 was not fleshed out and the detailed work was put off until the spring.

Missiles

The other issue is the announcement by President Bush that the United States will withdraw from the Anti Ballistic Missile Treaty of 1972, in order to test and build a missile defence system.

Here, those who want a vigorous approach to Russia have prevailed, and they hope that Russia will learn to accept this new reality.

The immediate signs are not unhopeful.

The Russian Defence Minister Sergei Ivanov, who met Mr Rumsfeld in Brussels this week, said afterwards that Russia would not "waste lots of money on an arms race."

He also dismissed the American "Son of Star wars" plan as a "myth" anyway.

It would not, he said, threaten Russia's security.

In the short-term, the American-led "war on terrorism" is likely to keep Russia and the United States on the same track.

Favours a closer relationship: Colin Powell

Russia has done well out of this - getting critics of its action in Chechnya largely silenced.

And America is happy to have public Russian support.

But there is still a basic uneasiness between the two - and the final outcome has yet to be determined.

Will Russia throw in its lot with the Western powers and economies, or will it continue to hover between Europe and Asia?