KABUL, Jan 10: US Vice President Joe Biden was in Afghanistan on Monday for a surprise visit aimed at assessing progress in handing over security from foreign to Afghan forces, a key issue that comes against a backdrop of mounting concerns in the US over the gains made in the nearly decade-long war.

The US is to begin withdrawing combat forces from Afghanistan in July, and questions remain about the ability of the country's security forces to take up the fight in the face of a virulent insurgency. Nato hopes Afghan forces will assume full responsibility for security by 2014.

Just a month ago, President Barack Obama came to Afghanistan telling US forces in the country they are making progress in their mission to defeat terrorism. American troops comprise the bulk of the 140,000-strong Nato force that has been battling the Taliban.

Nato and US officials have said they are making gains in quelling the fierce insurgency, but the Taliban has repeatedly shown its ability to regroup, despite constant offensives by the international coalition, and to carry out attacks across Afghanistan seemingly at will.

Last year was the deadliest for Nato forces in the country, with over 700 troops killed.

In an end-of-year review of the US strategy in Afghanistan, the Obama administration said the US had made advances in its push against the Taliban in southern Afghanistan — the insurgents' traditional stronghold — but acknowledged that “gains remain fragile and reversible”.

The White House said Biden, who last visited in January 2009, was to meet with President Hamid Karzai and US commander Gen David Petraeus as well as visit with US troops and civilian personnel. He was also to tour an Afghan Army training centre.

Karzai's relations with Washington have at times been rocky. His government is plagued by charges of corruption and US officials have expressed grave concerns about how this is affecting efforts to rebuild the country and bolster confidence in the young administration.

Social services are lacking, unemployment is rife and Afghans complain that they have seen little tangible improvement in their lives since the 2001 invasion that toppled the Taliban. The insurgents, in turn, have tried to capitalise on that frustration, tapping into the discontent to win new supporters.

Despite the ongoing tensions between Karzai and the US, the Afghan leader received a warm welcome during a trip to Washington in May, during which Biden said he thought Karzai was a trustworthy partner.

Biden is also reported to be heading to Islamabad this week to deliver a message that the US will send more help to Pakistan, which US and Afghan officials see as a key partner in routing the Taliban.

The Taliban's top leadership is believed to be hiding in Pakistan, but Islamabad has resisted pressure to crack down on them.

The challenges faced by Nato forces in Afghanistan were brought home by Nato's announcement on Monday that air strikes had killed three Afghan police officers and wounded three others mistaken for insurgents setting up an ambush.

The incident was at least the fourth in roughly a month in which coalition troops killed civilians or friendly forces in error, lapses that have threatened to further sour Afghan attitudes toward the foreign troops.

Kaykundi Deputy Governor Amanullah Gharji said the incident on Sunday happened when US Special Forces and local police had teamed up to hunt down Taliban fighters who had just carried out an attack. He said the strike may have been launched on the basis of a mistranslation by an interpreter with coalition forces.

The victims' families were initially outraged, asking why coordination had been so poor, he said.

“They said that they gave their men to fight shoulder-to-shoulder with the coalition forces and that they are against the insurgency,” Gharji said, adding that they wanted to know why the miscommunication occurred. The governor was able to defuse tensions by sending a representative to the area to explain the circumstances surrounding the attack.

Nato said a team on the ground called in air support after seeing “nine armed individuals setting up what appeared to be an ambush position”. The men later turned out to be Afghan police, it said. The incident was being investigated, Nato said.

In Kandahar, scene of some of the fiercest fighting of the war, a suicide car bomber struck a border police convoy on Monday, killing at least two officers and a civilian, said Zalmai Ayubi, the spokesman for the Kandahar provincial governor.

Nato says it is making progress in battling the Taliban with major offensives in the southern provinces of Kandahar and Helmand. An extra contingent of over 1,000 US Marines will be deployed in Helmand in what coalition spokesman Brig-Gen Josef Blotz said on Monday was a temporary operation to take advantage of gains on the ground.

The attack, which targeted officers working under the command of Abdul Razak, a powerful border police commander, took place near Spin Boldak.—AP