New US Defense Secretary Chuck Hagel landed in Afghanistan today 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 USD 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.

Dutch Prime Minister Mark Rutte announced today that military trainers from the Netherlands will withdraw earlier than planned from their mission in Kunduz province, saying local forces were ready to take over.

Rutte said some 200 other members of the 545-strong Dutch deployment in Afghanistan will remain as planned until next year.