The grandson of Afghanistan's last king pulled out of the country's presidential race on Wednesday and backed a candidate increasingly seen as the choice of outgoing President Hamid Karzai.

The withdrawal of Mohammad Nader Naeem narrows the field for the April 5 poll to eight and hints at further deal-making in the race to lead Afghanistan as NATO troops prepare to exit.

Taliban militants staged a gun and suicide attack on an Afghan election commission office in Kabul on Tuesday, killing two police officers, with less than a fortnight to go before the presidential poll.

The insurgents have vowed a campaign of violence to disrupt the ballot on April 5, urging their fighters to attack polling staff, voters and security forces in the runup to election day.

The Taliban on Monday vowed to target Afghanistan's presidential election, urging their fighters to attack polling staff, voters and security forces before the April 5 vote to choose a successor to Hamid Karzai.

Previous Afghan elections have been badly marred by violence, with at least 31 civilians and 26 soldiers and police killed on polling day alone in 2009 as the Islamist militants displayed their opposition to the U.S.-backed polls.

A magnetic bomb attached to a district governor's vehicle exploded in eastern Afghanistan on Saturday, killing the governor and his bodyguard, provincial authorities said.

The attack in Nangarhar province highlights the security threat surrounding presidential elections in a month's time, when voters will choose a successor to Hamid Karzai.

The brother of President Hamid Karzai dropped out of Afghanistan's election on Thursday and endorsed former Foreign Minister Zalmai Rassoul, an announcement seen as signaling the current leader's preferred successor.

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

At least 13,700 members of the Afghan security forces have died in fighting over the last 10 years -- nearly four every day -- according to new figures on government compensation paid to victims' families.

The grim death toll in the fight against Taliban-led insurgents was revealed by the Office of Administrative Affairs (OAA), the secretarial department that collates information for President Hamid Karzai and the cabinet.

President Hamid SKarzai has expressed "extreme anger" towards the United States as it prepares to end its 13-year war in Afghanistan, intensifying his criticism in his final months in power.

Karzai has taken an increasingly antagonistic view of the U.S. role in Afghanistan, despite formerly being a close ally and accepting billions of dollars in aid since he took office after the fall of the Taliban regime in 2001.

President Barack Obama told his Afghan counterpart Hamid Karzai on Tuesday he is now planning a full U.S. troop withdrawal, but did not rule out agreeing a post-2014 mission with the next Kabul government.

The U.S. threat was the latest twist in a long political tug-of-war with Karzai, who has infuriated Washington with his refusal to sign a Bilateral Security Agreement (BSA) governing a post-2014 mission in the dying months of his mandate.

Afghan President Hamid Karzai slammed Pakistan on Sunday for failing to "eliminate nests of terror" after the Taliban killed 20 soldiers and kidnapped seven in a daring raid on a border outpost.

The attack in the eastern Kunar province was the single deadliest strike on Afghan troops in recent months by the Taliban, and caused Karzai to cancel a planned two-day visit to Sri Lanka.

Afghan President Hamid Karzai on Tuesday condemned the killing of a former Taliban minister, saying the victim had recently returned from a Taliban meeting in Dubai where he had supported peace talks.

Abdul Raqeeb was gunned down in Pakistan's northwestern city of Peshawar on Monday as he left a religious seminary where he had been teaching.