A suicide bomber killed at least six Afghan soldiers in the suburbs of Kabul Thursday morning, officials said, the latest in a spate of Taliban attacks in the capital.

The bomber who was on foot targeted a bus carrying Afghan troops in Tangi Tarakhil on the outskirts of the capital, Kabul Crime Branch Unit chief General Farid Afzail told AFP.

Taliban spokesman Zabihullah Mujahid claimed responsibility for the attack in a message sent to the media.

"Around 7:15 am, a suicide bomber on foot targeted a bus of Afghan army soldiers in Tangi Tarakhil area of Kabul city," Afzail said.

"As a result of the attack, six Afghan army staff were martyred and 10 others were wounded."

A statement from the defence ministry confirmed the details of the attack and death toll, but said 11 people were wounded.

Kabul has been hit by a spate of deadly attacks in recent weeks, heightening concerns that Afghanistan could tip into a spiral of violence as the US-led military presence declines.

NATO's force in Afghanistan will change on December 31 from a combat mission to a support role, with troop numbers cut to about 12,500 -- down from a peak of 130,000 in 2010.

Thursday's attack came as Washington announced it no longer holds any prisoners in Afghanistan, a day after a damning US Senate report on the brutal treatment of "war on terror" detainees.

A US official confirmed to AFP that the last "third-country nationals" in US custody in Afghanistan had been transferred.

In March 2013, Afghan forces took control from the US of the notorious Bagram prison, renamed Parwan, located on the sprawling US military airbase, but American forces had remained in charge of non-Afghan prisoners.

The running of Bagram jail, 25 miles (40 kilometres) north of Kabul, was highly controversial, with rights groups accusing the US of abusing prisoners. A US army report found that two inmates were beaten to death in 2002.


'Rising violence'

The Senate report highlighted abuse at "black sites" around the world, several in Afghanistan including once known as the "Salt Pit" outside Bagram Air Base.

Afghan President Ashraf Ghani gave a stern condemnation of the CIA torture detailed in the report, saying it violated "all accepted norms of human rights" and were part of a vicious cycle of violence.

The report comes with the Taliban insurgency against Ghani's government still resilient, with a series of strikes in heavily-fortified Kabul.

Militants have targeted foreign guest houses, embassy vehicles, US troops and Afghan army buses in Kabul over the past month, undermining claims that the insurgency is weakening as NATO's 13-year war ends.

US Defense Secretary Chuck Hagel said on a visit to Kabul at the weekend that the upsurge showed "that the international community must not waver in its support for a stable, secure, and prosperous Afghanistan".

He said Afghanistan's newly-elected government and its security forces were ready to take charge of security.

Hagel insisted Afghanistan would not go the way of Iraq, where another US-trained army virtually collapsed in the face of an onslaught by Islamic State jihadists after American troops left the country in 2011.

But a day later, Afghanistan's chief executive Abdullah Abdullah told Britain's Sunday Times newspaper that Western forces were leaving prematurely.

"It is too abrupt," Abdullah told the broadsheet of pull-out plans.

"Two years ago we had 150,000 international troops and lots of jets and helicopters. Within two months there will be just 12,000. We need air support for the medical evacuation of casualties, intelligence and fast jets."