By Adrian Croft and Paul Taylor

LONDON, Nov 13 (Reuters) - Britain is unlikely to be able to send major reinforcements to Afghanistan because its armed forces are already overstretched, a leading opposition politician said on Thursday.

William Hague, the Conservative Party's foreign affairs spokesman, said "we'd take a lot of persuading" if Britain was pressed to reinforce its 8,100-strong force battling Taliban guerrillas.

U.S. President-elect Barack Obama has pledged to send more troops to Afghanistan, where the United States already has more than 30,000 soldiers, and he is expected to pressure European members of NATO to provide more troops and cash.

"Given the overstretch of our armed forces, it is unlikely to be possible to have a major increase in the number of our troops in Afghanistan," Hague told Reuters in an interview.

"We've done more than our proportionate share in Afghanistan and we continue to do so and should continue to do so, but it is time for the rest of NATO to step up to the plate in Afghanistan," said Hague, who visited Afghanistan in September with conservative leader David Cameron where they talked to British soldiers.

Asked about Hague's comments, a British Defence Ministry spokesman said the armed forces were stretched but that the situation was manageable.

"We remain confident that they are capable of meeting current levels of commitment, although we recognise that these levels cannot be sustained indefinitely," he said.

The British government has also called for a fairer sharing of the burden in Afghanistan although Prime Minister Gordon Brown said this week he was ready to "consider what's necessary" if Britain did receive a request for more troops.

The Conservatives have led Brown's Labour Party in the opinion polls for the last year, giving them a good chance of winning the next election which must be held by mid-2010.

AFGHAN TOLL RISES

Hague spoke as Afghan President Hamid Karzai held talks in London with Brown and the government said two Royal Marines were killed in Afghanistan on Wednesday, bringing the British military death toll there to 124 since 2001.

Britain has the second biggest foreign military contingent in Afghanistan. It also has 4,000 troops in Iraq, although most of those are expected to pull out next year.

U.S., British, Canadian and Dutch troops have shouldered much of the fighting in south and east Afghanistan, while other NATO members, notably Germany and France, have resisted U.S. pressure to operate outside the country's relatively safe north.

Hague criticised the British government and the European Union for agreeing this week to relaunch talks with Russia on a partnership pact, saying Moscow had not fully complied with a deal to end a war between Russia and Georgia over the breakaway South Ossetia region.

EU foreign ministers agreed to relaunch the talks after Britain and Sweden dropped their reservations.

"I just think it sends a signal of appalling weakness...to say we're resuming the partnership talks anyway," Hague said.

"A little more backbone in the Foreign Office on these things would...lead to sounder relations with Russia in the long term," he said. (Editing by Angus MacSwan)

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