US Defence Secretary Ashton Carter landed in Afghanistan Friday on an unannounced visit, as uncertainty lingers over President-elect Donald Trump's strategy on America's longest war with no end in sight.

Carter is expected to meet Afghan President Ashraf Ghani and American troops.

"#SecDef arrives in Afghanistan to visit troops, receive an update on efforts to support Afghan security forces & meet with senior officials," Pentagon spokesman Peter Cook said on Twitter.

Afghanistan got scarcely a passing mention in the bitterly contested US presidential election - even though the situation there will be an urgent matter for the new president.

Trump has given surprisingly little details on his expected foreign policy , with even fewer specifics on Afghanistan, where around 10,000 US troops remain after 15 years of conflict.

The Taliban are ramping up nationwide attacks despite the onset of winter, when fighting usually ebbs, even as international efforts intensify to jumpstart peace talks.

Carter landed at Bagram Airfield, the largest US military base in Afghanistan, where four Americans were killed in a suicide bombing in November, in a major breach of security.

The Taliban claimed responsibility for the bombing inside the heavily fortified base, north of the capital Kabul, which left 16 other US service members and a Polish soldier wounded as the insurgents step up attacks on Western targets.

"My assessment of our current capabilities (is that) we have adequate resources to conduct this mission at a moderate level of risk going forward ... this is acceptable for what we need to conduct," Nicholson told a Pentagon briefing.

James Dobbins, a former U.S. special envoy for Afghanistan, said Afghanistan would not factor highly for Trump given the fight against Islamic State militants in Syria and Iraq.

This, Dobbins said, was likely to mean that the number of U.S. troops in Afghanistan would remain unchanged, at least in the short term.

Trump will inherit a challenging security situation in Afghanistan. A number of provincial capitals have been under pressure from Taliban militants and Afghan forces have been suffering high casualty rates, with more than 5,500 killed in the first eight months of 2016.