The NATO Response Force (NRF), proposed by the United States two years ago, has now reached "initial operational capability", NATO Secretary General Jaap de Hoop Scheffer told an informal meeting of Alliance defense ministers in Romania.

The force currently comprises 17,500 troops, including air, land and sea units. But it is to expand into a 24,000-strong force able to deploy in as little as five days and operate without backup for up to a month.

"The NRF is at the heart of the ongoing transformation of NATO's military capabilities," De Hoop Scheffer said. "It will help to ensure that the Alliance can successfully conduct the full range of its ever more demanding missions."

The proposal for such a force was made by Washington two years ago, as part of the transformation of the US-led North Atlantic Treaty Organization, accelerated by the September 11, 2001, attacks.

The NRF is expected to reach fully operational capability in two years' time, De Hoop Scheffer told the ministers, gathered for two days of talks in the Carpathian mountain ski resort of Poiana Brasov.

He noted that elements of the NRF had already been deployed. These included units sent to protect the Olympic Games in Athens inaugust and an Italian batallion which was currently in Afghanistan.

But he also conceded that the force's effectiveness would be limited by NATO's requirement for unanimity in any decision to deploy it.

"The NRF will go where the Allies send it," he said.

The requirement for unanimity would for example make it virtually impossible to send the force to Iraq because France and other countries which opposed last year's US-led war on the oil-rich country would be unlikely to give their consent.

The NRF notably includes forces able to tackle the fallout from weapons of mass destruction, whether chemical, biological or nuclear, and to support counter-terrorism operations.

The force could also be used to evacuate non-combatants from crisis areas, as well as more traditional NATO peacekeeping duties.

De Hoop Scheffer told the ministers that NATO had achieved much in transforming itself in the last two years, during which time it has launched its first ever operations outside Europe.

But the Dutch NATO head said the 26-member Alliance had "less reason (for) satisfaction (about) marshalling forces for operations".

NATO took over the International Security Assistance Force (ISAF) in Afghanistan last year but has struggled to meet a commitment to expand it from Kabul into the north of the country.

The US was Wednesday pressing its NATO counterparts to do more in Afghanistan.

US Defense Secretary Donald Rumsfeld wants NATO to extend ISAF into western Afghanistan and to integrate the international and US military forces in the central Asian country.

"We must do more... We must use this meeting to give a strong impulse to that work," De Hoop Scheffer said.

The Romanian meeting was also expected to discuss an expanding NATO mission to train Iraqi security forces. The Alliance agreed details of the plan last week and the US is keen to press ahead with the operation ahead of elections in the violence-wracked country that are scheduled to take place in January.