US Defense 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 on his last official trip to Afghanistan before he hands over the reins to Trump's pick for Pentagon chief, the hardline retired general James Mattis.

Carter's visit comes as concerns mount over growing insecurity in Afghanistan, where around 10,000 US troops are assisting struggling Afghan forces to combat a resilient Taliban insurgency along with Al-Qaeda and Islamic State militants.

"#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 in the conflict-torn country 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 how he will tackle the war in Afghanistan.

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.

Fifteen years and hundreds of billions of dollars since the US-led invasion of Afghanistan in 2001, the security situation in the country remains fraught and Afghan security forces are struggling to contain the conflict.

One of the most important questions facing Trump on Afghanistan is how many American troops will stay in the country, observers say.

President Barack Obama was forced to slow a planned withdrawal of US troops in the face of Taliban gains, and about 8,400 will remain in the country when he leaves office early next year.

Mattis, whose nicknames in the military include "Mad Dog" and the "Warrior Monk", has led troops in Afghanistan and has previously criticised Obama's plan to pull forces from the country.