By Sayed Salahuddin

KABUL (Reuters) - Technocrats and some existing ministers will be appointed to Afghan President Hamid Karzai's new government, a spokesman said on Wednesday, but his main rival branded his re-election illegal and ruled out taking any part.

Karzai, returned to power after a needless presidential run-off vote was abandoned on Monday, is under pressure from his Western backers to root out the corruption that tainted his previous administration, with a top U.S. military official saying he should prosecute wrongdoers to shore up his legitimacy.

The run-off, triggered after widespread fraud marred the first round in August, was canceled after Karzai's only rival, former Foreign Minister Abdullah Abdullah, withdrew citing serious concerns about the vote.

That decision came after weeks of political uncertainty, while U.S. President Obama also weighs whether to send up to 40,000 more troops to fight a resurgent Taliban, who had threatened to disrupt the poll and branded Karzai's return as a farce.

Karzai has committed himself to an inclusive government, but Abdullah ruled out taking any part despite pressure for a power-sharing deal.

"I have no interest in the future cabinet of Karzai's government and I will pursue my agenda, which is change," Abdullah told a news conference at his Kabul home.

Abdullah said the government-appointed Independent Election Commission's (IEC) decision to cancel the November 7 run-off was illegal and said Karzai would not be able to deliver reform.

"A government which is derived from such an illegal decision will not be able to deliver," Abdullah said in his first public comments since the IEC's decision.

BRITISH TROOPS SHOT

An Afghan policeman shot dead five British soldiers at a checkpoint in southern Afghanistan on Tuesday, the British Ministry of Defense said, underlining security concerns and certain to fuel debate in Britain about its presence.

The NATO-led International Security Assistance Force in Afghanistan said six British troops and two Afghan police were also wounded. British Prime Minister Gordon Brown said the Taliban may have infiltrated the police.

A spokesman for the Taliban said the policeman was a member.

"It's a new Taliban tactic, we have more Taliban who have infiltrated into the police and army," Taliban spokesman Qari Mohammad Yousuf said.

The flawed electoral process and climbing death tolls have undermined support for the war in the United States, Britain and other Western countries with troops in Afghanistan. Western leaders say they expect Karzai to root out corruption and appoint more competent and credible figures to his new government.

"The government the president has in mind is one that will have special places for experts, educated and professional people," Karzai spokesman Siyamak Herawi told Reuters.

"It will have new figures and some of the old ministers who have done well," he said.

Herawi said Karzai would announce his new government within three weeks. Washington and others have expressed concern Karzai would give prominent places to regional chieftains and former guerrilla commanders who backed him in the election.

Admiral Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, said Washington was "extremely concerned" about current levels of corruption in Afghanistan, adding that a legitimate government in Kabul was key to any counterinsurgency strategy.

"He's got to take concrete steps to eliminate corruption. That means that you have to rid yourself of those who are corrupt, you have to actually arrest and prosecute them. You have to show those visible signs," he told a Washington conference.

"If we don't get a level of legitimacy and governance, then all the troops in the world aren't going to make any difference."

Obama and European leaders have backed Karzai but have also said he must work harder on good governance, respect for human rights, economic development, battling the drug trade and training Afghan security forces so that foreign troops can leave.

"Given what the international community is emphasizing on the eradication of drugs and corruption, all these issues will be taken into consideration," Herawi said.

Afghanistan is the world's leading producer of opium, used to make heroin, a trade that helps fund the Taliban-led insurgency.

There are nearly 110,000 Western troops in Afghanistan, two-thirds of them American. The White House has said a decision by Obama on future troop levels is still weeks away.

(Additional reporting by Hamid Shalizi and Yara Bayoumy in KABUL, Peter Griffiths in LONDON, and Adam Entous in WASHINGTON; Writing by Paul Tait; Editing by Alex Richardson)