Vice President Joe Biden was in Afghanistan Monday for a surprise visit to assess progress in the key issue of handing over security from foreign to Afghan forces, the White House said.

Biden's trip comes against a backdrop of mounting questions in the U.S. about the pace of progress in the nearly decade-long war. The U.S. 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 U.S. 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 U.S. 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 U.S. strategy in Afghanistan, the Obama administration said the U.S. 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 U.S. commander Gen. David Petraeus as well as visit with U.S. troops and civilian personnel. He was to also tour an Afghan Army training center.

Biden is also reported to be heading to Islamabad this week to deliver a message that the U.S. will send more help to Pakistan, which U.S. 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 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. Karzai has repeatedly complained about civilian deaths in NATO operations as the coalition tries to stamp out the persistent Taliban insurgency.

Kaykundi Deputy Gov. Amanullah Gharji said the incident Sunday happened when U.S. Special Forces and local police had teamed up to hunt down Taliban fighters who had just carried out an attack in the central province. 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.

There have been a string of such recent tragic mishaps. On Dec. 24, troops with the coalition killed two private security guards during a raid on what turned out to be company compound after receiving intelligence that insurgents were planning an attack on the U.S. embassy.

A day earlier, a NATO helicopter engaged in a hunt for suspected militants opened fire on a convoy of cars heading to an event hosted by the head of a local council, killing a police officer and the brother of a former lawmaker. Earlier that week, a clash between NATO forces and insurgents left five civilians dead in Helmand.

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 U.S. Marines will be deployed in Helmand in what coalition spokesman Brig. Gen. Josef Blotz said Monday was a temporary operation to take advantage of gains on the ground.

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

The attack, which targeted officers working under the command of Abdul Razak, a powerful border police commander, took place near Spin Boldak. The town near the Pakistani border was also where another suicide bomber on Jan. 7 blew himself up in a bathhouse, killing 17 people including Razak's deputy.

"They attacked my deputy, and now they are attacking my people," Razak said. "But I will root them out of my area."

The Taliban has claimed responsibility for both attacks in the town. Even as they are being squeezed in the south, however, the insurgents have expanded their operations elsewhere in the country.

In the north, NATO said coalition and Afghan forces killed more than 10 insurgents in a raid in Kunduz province that targeted a Taliban leader said to have close ties to the movement in Pakistan.

Two other suspected insurgents were detained, the coalition said. The operation was at least the second in as many days in Kunduz province, where the Taliban is particularly active.

Kunduz's provincial police chief, Gen. Abdul Rahman Sayedkhaili, said 13 were killed in the operation, including a Taliban district chief and one of his senior commanders. But it was not clear if the targeted leader was among those killed or detained.

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Associated Press writers Mirwais Khan in Kandahar and Amir Shah and Rahim Faiez in Kabul contributed.