LONDON: The West could withdraw support for Afghan President Hamid Karzai if he fails to live up to its expectations during his second term, Prime Minister Gordon Brown warned on Friday.

While defending Britain's Afghanistan mission against waning public support, Brown admitted Karzai's government had become a "by-word for corruption" -- and also pressed for more Afghan troops to bear the brunt of frontline action.

He said he had spoken to Karzai, whose re-election was confirmed this week, several times in recent days and urged progress on the key issues of security, governance, reconciliation, economic development and regional relations.

"If the government fails to meet these five tests, it will not only have failed its own people, it will have forfeited its right to international support," Brown said in a speech in London.

He said he had proposed Karzai draw up a new anti-corruption law, with a new commission advised by a high-profile international figure to investigate and prosecute graft, and new rules for the transparent awarding of contracts.

He also gave a warning on Karzai's government appointments, saying: "Cronies and warlords should have no place in the future of a democratic Afghanistan."

"Sadly, the government of Afghanistan has become a by-word for corruption. And 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.

It has been a bloody week for British troops in Afghanistan. Seven soldiers have been killed since last Saturday, five shot by a rogue Afghan policeman, bringing to 230 the number killed since the US-led invasion in October 2001.

Brown repeated that the mission was to stop Al-Qaeda from using Afghanistan as a base for attacks on Britain and elsewhere, and said it "must not fail".

"We cannot, must not and will not walk away," he said.

However, he said more Afghan troops were needed, adding: "The first priority of any government is to provide security for its people. It is not sustainable to subcontract that task indefinitely to the international community.

"So the expansion and training of the Afghan army and police must be the new government?s first priority."

About 500 British troops are involved in mentoring Afghan soldiers and policemen to help them take over security from international forces, but this role was thrown into question by the shooting on Tuesday.

An Afghan policeman opened fire at a checkpoint in southern Helmand province -- where the vast majority of Britain's nearly 9,000 troops are based -- leaving five British soldiers dead.

Brown expressed his sorrow at the loss but insisted: "We will not give up this strategy of mentoring. Because it is what distinguishes a liberating army from an army of occupation."

The killings have added to public unease about the Afghan mission.

A YouGov/Channel 4 News survey Thursday found 57 percent of people thought British troops were not winning their fight against Taliban insurgents, and "victory is not possible," an increase from 48 percent just two weeks ago.

Thirty-three percent think the war is being won, or that victory is possible eventually. Yet just 20 percent think troops should stay until the job is done.