Britain will not expand its role in Afghanistan to take over from departing U.S. troops, Britain's defence secretary said on Thursday in a speech which also addressed spending, the cost of the Libya campaign and the role of women on submarines.

After more than a decade of war in Afghanistan, the White House is moving ahead with plans to withdraw 33,000 of its near 100,000 troops by next September, raising speculation among some military analysts that British soldiers might be used to pick up any slack.

I'm clear that we will be able to manage with whatever drawdown pattern the U.S. finally determines upon .... critically, we will not be expected to backfill for American troops, Defence Secretary Philip Hammond told the Royal United Services Institute defence thinktank in London.

U.S. commanders understand that, people in the White House and the Pentagon understand that. We could not have been clearer, he added.

Britain has about 9,500 troops in Afghanistan, mainly in southern Helmand province, which has seen some of the deadliest fighting between Western forces and Taliban and other insurgents.

Hammond said that while U.S. troops may be drawing down, Afghan forces were increasingly able to take over security responsibility, and he had received assurances that U.S. support, such as warplanes and helicopters, would remain.

Britain and other Western countries with troops in Afghanistan have said they are committed to handing over responsibility for security to Afghan authorities by the end of 2014.

The amount of troops, equipment and cash contributed to the Afghan war effort has been a source of tension among some states in Europe, with Britain urging its NATO partners to do more.

Hammond said he would increase those efforts, particularly as the United States puts more focus on its ties with Asia, but he admitted getting more European commitment to defence would be difficult given the region's financial difficulties.

Over the long term we have to count upon a lower level of U.S. military commitment in Europe as their attention swings elsewhere, he said.

SUBMARINES, LIBYA

Britain last year slashed its defence budget by about eight percent in real terms over four years, cut military jobs and scrapped hardware programmes as part of the Strategic Defence and Security Review (SDSR), an effort to slim down the military to tackle a big budget deficit.

Hammond, who took over as defence secretary in October after the resignation of his predecessor Liam Fox, said he would not reopen the review.

Let us be under no illusions -- unpicking the SDSR piece-by-piece is simply not an option, he said.

Hammond also announced the overturning of a ban on women serving in submarines, and gave a final estimate of the cost to Britain of its involvement in the bombing campaign over Libya.

The Ministry of Defence said women had earlier been excluded from working in submarines due to concerns about higher levels of carbon dioxide being a risk to female health, but research has since shown the fear to be unfounded.

Hammond said operations in Libya, where Britain contributed to a NATO-enforced no-fly zone, cost 212 million pounds, down from an earlier estimate of 260 million pounds.

(Reporting by Mohammed Abbas; Editing by Matthew Jones)