By Barnaby Mason BBC Diplomatic Correspondent

An American Deputy Secretary of State, Richard Armitage, says Washington would like to see Nato admit as many new members as possible and has urged the candidates, including Bulgaria and Romania, to redouble their efforts.

Mr Armitage was speaking in Bucharest at a meeting of 10 aspiring member states. A Nato summit in November will decide whom to invite to join.

The aftermath of 11 September has increased the strategic importance of several areas of the world in Washington's eyes, and Nato's south-eastern flank is one of them.

Bulgaria and Romania were seen as lagging far behind other Nato candidates like the Baltic states and Slovenia; now they seem more plausible.

Air space

Both countries seized the opportunity to show how useful they could be in President Bush's war on terrorism.

They have opened their air space and military bases to the Americans for operations in Afghanistan, and the Romanians have sent peacekeeping troops to Kabul.

In an apparent reference to possible future action against Iraq, the Bulgarian Foreign Minister, Solomon Pasi, told the Washington Post that the next time the United States needed support, Bulgaria would be an excellent ally.

The two countries argue that their membership would fill the hole in Nato between Hungary and Turkey, help stabilise the Balkans and make it easier to protect oil pipelines.

Finishing line

All this has not gone unnoticed. Mr Armitage praised the efforts of Bulgaria and Romania to help.

He said the Bush administration was looking for the widest possible expansion of Nato and encouraged them to sprint to the finishing line.

But Mr Armitage also emphasised that no decision would be made until October, the month before the Nato summit in Prague.

He said the candidates should redouble their efforts on democratic reform, fighting corruption and re-organising their armed forces.

Both Greece and Turkey are actively backing the admission of Bulgaria and Romania.

Nato will make a collective decision.

But for the United States at least, long-standing doubts about their political qualifications and the value of their military contribution now carry less weight.