London - Afghan President Hamid Karzai risks the withdrawal of international support if his government should fail to improve security and root out corruption, Prime Minister Gordon Brown warned Friday.

"I am not prepared to put the lives of British men and women in harm's way for a government that does not stand up against corruption," Brown said in a speech in London.

He warned that if Mr Karzai's government failed to fulfil these requirements, "it will have forfeited its right to international support."

He listed the need for the Kabul government to improve relations with its neighbour Pakistan as being among the requirements Karzai was expected to meet.

Commentators said Brown's speech was the clearest warning yet by Britain that the mission in Afghanistan could fail.

But Brown, speaking after the shock murder of five British soldiers by an Afghan policeman in southern Helmand, also again underlined the importance of the Afghan mission in protecting Britain and other nations from terrorism.

Britain could "not afford to walk away" from the mission in Afghanistan, Brown said in a hastily arranged speech at the Royal College of Defence Studies in London.

Recent opinion polls have shown a drop in popular support in Britain for the Afghan mission, in which 230 British soldiers have died since 2001. (dpa)