Afghan leaders in Bonn signed a UN-brokered peace agreement on Wednesday and created an interim administration as an errant US bomb caused the worst US military combat deaths of the war.

A stray bomb, a "smart" munition dropped by a B-52 bomber aimed at Taliban forces north of their stronghold of Kandahar, killed three US servicemen and injured 19 others, members of two detachments of US special forces. Five Afghans fighting alongside the US were also killed.

The deaths came during what US officials have predicted would be the most dangerous phase of the war.

Fighting continued around Kandahar and US-backed Afghan forces moved into cave complexes around Tora Bora, where members of Osama bin Laden's al-Qaeda network are believed to be hiding.

The interim administration will be led by Hamid Karzai (pictured), a Pashtun tribal leader who re-entered the country in October to rally opposition to the Taliban. It is expected to take over Kabul from the Northern Alliance on December 22. There were reports on Wednesday that he had been injured in the "friendly fire" incident, but he told Britain's Channel Four news that he was not caught up in it.

Diplomats hailed the agreement, which came after a week of negotiations complicated by the unwillingness of some Northern Alliance leaders, particularly former president Burhanuddin Rabbani, to cede power. Many warned, however, the real test would come once the government began its work in Kabul.

Some claimed ethnic groups, especially the majority Pashtuns who dominated the Taliban regime that ruled in Kabul from 1996, were under-represented.

Lakhdar Brahimi, the UN's special envoy to Afghanistan who chaired the talks, said: "No ethnic group is happy, because there are no precise statistics and every ethnic group thinks that they are much more numerous than everybody else thinks."

Emphasising this potential for disagreement, there were reports on Thursday morning that the ethnic Uzbek warlord General Abdul Rashid Dostum was planning to boycott the interim government to be installed in Kabul.

"We announce our boycott of this government and will not go to Kabul until there is a proper government in place," he was reported as saying.

The administration's top posts went largely to a younger generation of Northern Alliance leaders, including Abdullah Abdullah, its foreign minister, Yunus Qanuni, the interior minister, and Mohammed Fahim, the defence minister and the military commander of the Northern Alliance.

Delegates loyal to Mohammad Zahir Shah, Afghanistan's former king, garnered the other key posts, including those of the finance and reconstruction ministries. Two women are included in the cabinet.

Mr Karzai was chosen as leader of the six-month long administration after pressure from the US and UN.

The 30-member group will give way to a two-year transitional government established by a Loya Jirga, or council of elders. The UN will play a supporting role and help Afghanistan with its reconstruction, which is expected to cost $7bn-$12bn (E7.8bn-E13.5bn) in the first five years.

The deaths bring to four the number of Americans killed in combat in Afghanistan, the other being a Central Intelligence Agency operative.