Newly appointed US Defence Secretary Chuck Hagel has arrived in Kabul for his first trip abroad after being sworn-in to the position last month. 

Hagel arrived in Afghanistan's capital on Friday with a plan to meet US commanders and Hamid Karzai, the Afghan president.

He said he would also make his own assessment about the United States' longest war as it enters its final stretch.

Hagel's visit to Kabul comes after Karzai has taken steps to limit US military activities. 

"I need to better understand what's going on," said Hagel adding that the purpose of the unannounced visit was to "make my own assessment and listen to our commanders". 

He said that the visit would be the first trip to Afghanistan since a mid-2008 visit with then-senator Barack Obama during Obama's campaign for presidency.

Hagel, an early Republican critic of the Iraq war, will help determine how large a residual mission will be kept there once NATO troops concludes its combat mission at the end of next year. 

James Mattis, the outgoing head of the US military's Central Command, said on Tuesday that he recommended keeping 13,600 American troops in Afghanistan, which is more troops than considered by the White House and NATO defence chiefs. 

"I think it is important [that] General Mattis - all of our commanders - have an opportunity for their input. The president wants that, needs that, welcomes that," said Hagel, adding that the president had not made a final decision. 

"If you look at the role that we've had for the past 12 years as the lead combatant in Afghanistan, that's a totally different role than what we're transitioning into," said Hagel discussing the role of the US mission after Afghans take charge of their security. 

"I think we are transitioning in a way that give the Afghan people a very hopeful future."

Obama announced the withdrawal of 34,000 US troops by early 2014. 

Source: Agencies