US Defence Secretary Carter says slower troop withdrawal from Afghanistan being considered

US President Barack Obama's new Pentagon chief said the United States was seriously considering slowing the pace of a troop withdrawal from Afghanistan, as the country battles a resilient Taliban insurgency.

US Defence Secretary Ashton Carter's comments in Kabul on Saturday offered the clearest sign yet that Washington was ready to delay the closure of some bases and retain more troops after appeals by Afghanistan's new President Ashraf Ghani and advice from commanders.

To safeguard "hard-won" progress, Obama "is considering a number of options to reinforce our support for President Ghani's security strategy, including possible changes to the timeline for our drawdown of US troops," Carter said after talks with Afghan leaders.

"That could mean taking another look at the timing and sequencing of base closures to ensure we have the right array of coalition capabilities," he said at a news conference with Ghani.

Apart from troop numbers, the United States and its allies would need to make "long-term commitments in resources, equipment and other support" to ensure the success of the Afghan forces, he said.

Carter's visit comes amid a sharp rise in Afghan casualties from the 13-year conflict, with the UN recording a 22 per cent increase in the number of civilians killed and injured in 2014 from the previous year due to a spike in combat between government and insurgent forces.