KABUL - Afghanistan's former presidential challenger on Wednesday slammed Hamid Karzai's re-election as illegal, piling pressure on the head of state as his foreign allies warned him to deliver on reform pledges.

Three days after quitting a scheduled run-off, Abdullah Abdullah said the subsequent decision by the Independent Election Commission (IEC) to hand Karzai another five years in power had no basis in law and underlined its bias.

"This (IEC) decision does not have a legal basis," the former foreign minister told reporters, albeit refraining from calling on his supporters to take to the streets in protest.

"Such a government which lacks legitimacy cannot fight corruption.

"A government which comes to power without the people's support cannot fight phenomena of terrorism, unemployment, poverty and hundreds of other problems."

Abdullah delivered a withering verdict on Karzai's rule, saying the Afghan people had hoped to open a new chapter in the impoverished, conflict-torn country but instead the president had blown a "golden opportunity".

Reiterating earlier calls for his supporters not to resort to violence, he said: "Let there be only one example of law-breaking."

Abdullah also said he was not seeking any position in Karzai's government nor did he make any demands for his supporters.

"I have no interest in joining the cabinet," he said.

In his first appearance since his re-election, Karzai vowed Tuesday to "eradicate the stain" of corruption as well as pledging to form a government reflective of the whole of Afghanistan, historically riven by ethnic rivalry.

US President Barack Obama tempered his congratulations to Karzai with a call for him to embark on a new drive to crack down on widespread corruption which has helped sour relations between Washington and Kabul.

Obama is mulling a request from his top commander on the ground for tens of thousands of reinforcements.

The killing of five British soldiers by an apparent "rogue" Afghan policeman at a checkpoint, which London announced Wednesday, underscored the increasing danger and complexity of the war in Afghanistan.

"I have spoken to the interior minister (Hanif Atmar) who shares my deepest regret for this incident and he gave me his assurance that this incident will be fully and transparently investigated," said Stanley McChrystal, the US general commanding coalition troops.

In a further sign of the international pressure on Karzai, the head of the UN mission in Afghanistan warned the president that the world could desert him if he does not show his commitment to reform in his next cabinet.

"I think the debate we've seen over the last few months in the international community will become even more heated, even more difficult, if we do not have an important, positive signal given through the composition of a new government," Kai Eide told US public broadcaster PBS.

"Some Afghans believe that Afghanistan is of such strategic importance that we will stay here whatever happens. It is simply not correct."

More than 100,000 US and NATO forces are currently in Afghanistan to combat a Taliban insurgency that has gathered pace in recent months, eight years after the Islamists were toppled from Kabul and then replaced in power by Karzai.

In a sign of wariness among the coalition, a former British minister who had responsibility for Afghanistan said London should withdraw its troops.

"It would be better to bring home the great majority of our fighting men and women and concentrate instead on using the money saved to secure our own borders, (and) gather intelligence on terrorist activities inside Britain," Kim Howells wrote in the Guardian newspaper.

Britain has 9,000 troops in Afghanistan, the second largest contingent after the US, and the mounting death toll has prompted a sharp decline in public support for the mission.