KABUL: At least six Nato soldiers were killed when a suicide bomber on a motorbike attacked a joint US-Afghan patrol near Bagram air base in Afghanistan on Monday.

Brig Gen William Shoffner, head of public affairs at Nato's Resolute Support base in the Afghan capital Kabul, said that three foreign troops were also wounded in the attack.

However, in a tweet, Taliban spokesman Zabihullah Mujahid claimed responsibility and said 19 US soldiers had been killed and a number wounded.

The Nato headquarters in Kabul confirmed there had been a vehicle-borne improvised explosive device attack near Bagram Air Base and said it was under investigation.

Shoffner said the attack happened at around 1.30 pm local time in the vicinity of the base, which is the biggest US military facility in Afghanistan. Nato could not confirm the nationality of the dead, according to policy.

Bagram District Governor Abdul Shukur Qudusi also confirmed the incident and the number of casualties.

The police chief of Parwan province said three Afghan policemen had been wounded in the attack.

He said he was not authorised to give details on foreign troop casualties.

Bagram, around 40 km to the north of the Afghan capital, Kabul, is one of the main bases for the 9,800 US troops left in Afghanistan after international troops ended combat operations last year.

The attack comes just over a week after suicide attacks on Kandahar air base in southern Afghanistan and on a Spanish embassy guest house in Kabul, underlining the Taliban's ability to hit high-profile targets linked to the U.S.-backed government.

On Monday, Taliban forces in Helmand closed in on the district of Sangin as they tightened their grip on the volatile southern province.