US Defense Secretary Hagel in Kabul on surprise visit

New US Defense Secretary Chuck Hagel landed in Afghanistan on Friday on an unannounced visit nine days after he was sworn into office, vowing to ensure a successful withdrawal of international troops.

Hagel arrived in Kabul as the US-led military coalition prepares to pull out by the end of next year and leave Afghan security forces to battle the Taliban insurgency that has raged across the south and east of the country.

"We have a lot of big issues and challenges ahead as we prepare for a responsible transition," he told reporters on his plane. "That transition has to be done right, it has to be done in partnership with the Afghans (and) with our allies."

Hagel was sworn in last week as heavy cuts loom for the US military, but he said that Americans realised that Afghanistan remained a major conflict zone with US troops fighting against Islamist militants since the 9/11 attacks.

"We have 66,000 troops still at war in a combat zone, that reality is there," he said. "I don't minimise or marginalise anything just because we may be transitioning to a new phase, we're still at war in Afghanistan."

Since taking the helm at the Pentagon, Hagel has expressed concern over the Defense Department's budget being slashed by roughly $46 billion under the automatic cuts triggered by political deadlock in Washington.

"I'm going to Afghanistan first to thank our troops and acknowledge their work," he said on the plane. "It's always important when a new leadership comes in... that we recognise the people who are the ones on the frontlines."

A total of 100,000 international troops are currently deployed in Afghanistan, with all combat forces due to exit by the end of 2014.

US President Barack Obama last month announced that 34,000 US troops would withdraw in the next year, halving the size of the current 66,000-strong US force.

Afghan forces are assuming control of security across the country, but there are widespread fears that they will be unable to face down the Taliban and that the country could tip into greater instability.

In a sign of the continuing violence, a NATO civilian contractor was killed by three people wearing Afghan security force uniforms on Friday in the latest suspected "insider attack" to target the coalition.

More than 60 foreign soldiers were killed in 2012 in "insider attacks" that have bred mistrust and threatened to derail the training process.

During his visit to Kabul, Hagel will meet with President Hamid Karzai, the US-backed leader who has had an often rocky relationship with Washington since he came to power after the fall of Taliban in 2001.

Karzai will step down next year before elections and the US is keen to ensure a fair poll that brings in a stable government as international operations wind down.

"The goal we have established -- to have Afghans assume full responsibility for security by the end of 2014 -- is clear and achievable," Hagel said in a message to US troops.

Among the subjects likely to be raised in talks with Karzai is the size of a residual US force to remain behind after the formal withdrawal to continue training Afghan soldiers and to conduct counterterrorism missions.

"It was never the intention of the United States to stay in Afghanistan indefinitely," Hagel said. "It is the Afghan people who need to make and will make their own decision about their future."