LONDON: Britain and the United States on Friday defended their strategy in Afghanistan amid mounting criticism over the rising death toll from war-weary voters.

But a keynote speech by British Prime Minister Gordon Brown was overshadowed by the resignation of a ministerial aide in protest at London's stance in Afghanistan, where 212 British soldiers have been killed in the campaign.

In Washington, US Defence Secretary Robert Gates insisted on Thursday that the war was not "slipping through the administration's fingers," but admitted: "There is a limited time for us to show that this is working."

"We are mindful of that, we understand the concerns of many Americans in that area but we think that we now have the resources and the right approach to start making some headway," Gates told reporters.

The bloodshed mounted Friday as up to 90 people were killed in a NATO air strike on two fuel tankers hijacked by the Taliban, while a French soldier was killed and nine others wounded in a bomb attack on their convoy.

In London, Brown pledged that Britain will not walk away from Afghanistan when its own security was at stake.

"People ask what success in Afghanistan would look like. The answer is that we will have succeeded when our troops are coming home because the Afghans are doing the job themselves," he said, according to excerpts released by his office.

His message was clouded by the resignation late Thursday of Eric Joyce, a parliamentary aide to Defence Secretary Bob Ainsworth, who warned there were problems in Afghanistan "which need fixing with the greatest urgency".

Joyce, a lawmaker and former army major, also criticised NATO allies in Afghanistan, saying many of them "do far too little," leaving Britain to shoulder more of the combat role.

He called for more "geopolitical return from the United States for our efforts," adding: "For many, Britain fights; Germany pays; France calculates; Italy avoids."

After a fraud-tainted first round last month which has left much-criticised President Hamid Karzai edging towards re-election, Joyce also urged better communication of the government's rationale for war.

"I do not think the British people will support the physical risk to our servicemen and women unless they can be given confidence that Afghanistan?s government has been properly elected," he said.

Brown has faced growing questions over the Afghanistan mission amid a surge in British troop deaths which has sparked a row over whether soldiers have adequate resources to combat Taliban insurgents.

"When the security of our country is at stake we cannot walk away," Brown said in the speech at the Institute of Strategic Studies think-tank in London.

Finance minister Alistair Darling said it was vital to support the governments in Afghanistan and Pakistan.

"Otherwise what will happen is, if Al-Qaeda become re-established there and can operate from that area with impunity, it will come back and it will affect us here in Britain."

The comments came after officials said Thursday two more soldiers were killed in southern Afghanistan, taking the British toll to 212 since 2001 when a US-led invasion ousted the hardline Taliban regime.

US Joint Chiefs of Staff chairman Admiral Michael Mullen stressed that "there is a sense of urgency" and "time is not on our side" in Afghanistan -- but he rejected some commentators' suggestions that US troops withdraw now.

"There's no way to defeat Al-Qaeda, which is the mission, with just that approach, you can't do it remotely, you can't do it offshore," Mullen said. "I certainly don't think it's time to leave."