The brother of President Hamid Karzai on Thursday dropped out of Afghanistan s election and endorsed former foreign minister Zalmai Rassoul, an announcement widely seen as signalling the current leader s preferred successor.

The first major move in the election campaign before voting on April 5 came as a US air strike in the east of the country accidentally killed five Afghan soldiers.

Relations between the US and Afghanistan have become severely strained as NATO combat troops prepare to withdraw by the end of this year, and the latest "friendly fire" incident is set to worsen ties further ahead of the election.

The new president will face a testing term in office as the final 55,000 NATO troops exit and the Afghan army and police are left to fight the Taliban insurgency alone, 13 years after the Islamists were ousted from power.

"I and my team, we consider ourselves as a key part of this new alliance and declare my support for Doctor Zalmai Rassoul," Qayum Karzai told a press conference.

Rassoul, a softly-spoken loyalist of Hamid Karzai, said: "From now on, both teams will fight for victory... and I m sure with the blessing of God, and the support of the people, we will win."

President Karzai, who is constitutionally barred from standing again after serving two terms, has pledged not to publicly endorse any candidate in the elections.

But his brother s support for Rassoul is likely to be taken as a signal that Rassoul is the president s choice.

One senior diplomat told AFP that Rassoul s campaign was being "propelled" by the palace.

Qayum Karzai s withdrawal leaves 10 candidates in the race, including opposition leader Abdullah Abdullah, who came second in the chaotic and fraud-riddled 2009 election, and former finance minister Ashraf Ghani.

The NATO combat mission in Afghanistan ends in December, though a small number of US troops may stay on for training and counter-terrorism operations if a long-delayed security deal is signed with Washington.

More than a decade after the fall of the Taliban in 2001, Afghanistan remains in a parlous condition, despite billions of dollars of military and development aid pouring into the country.

Afghanistan and United States have engaged in a series of public disputes, with President Karzai launching regular verbal assaults against Washington as the NATO withdrawal gathers pace.

NATO offered its condolences over the five deaths in the eastern province of Logar and vowed action to avoid further mistakes.

Khalilullah Kamal, the Charkh district governor, told AFP he had visited the site of the attack, which he said was from a US drone.

"The post is totally destroyed," he said. "The Americans used to be in that post but since they left, the ANA (Afghan army) took over. The post is on a hilltop. The attack was conducted by drones."