Image copyright Getty Images Image caption The base at Musa Qala was under the control of Afghan forces before Wednesday's attack

Taliban fighters have seized control of a strategically-important district headquarters in Afghanistan's Helmand province despite American air strikes.

Musa Qala, once a key Nato position, is the second town in northern Helmand to fall to the Taliban in recent weeks.

The group also captured Nawzad after intensifying attacks in the region.

Musa Qala saw some of the fiercest clashes between Western forces and the Taliban following the American-led invasion of Afghanistan in 2001.

Three US air strikes on Saturday around Musa Qala reportedly killed up to 40 Taliban fighters but the militants regrouped to push Afghan ground forces back.

District governor Mohamad Sharif said he fled the district on Wednesday morning as the Taliban attacked.

"We left the district early in the morning because the Taliban were attacking from all sides," he told Reuters.

"We had asked for reinforcements for days but none arrived and this was what happened."

In southern Helmand, also on Wednesday, two Nato soldiers were killed when two men in Afghan military uniforms opened fire on a vehicle at a military base.

The nationalities of the victims are not known.

Key strategic position

Musa Qala was once a Taliban stronghold and centre of the country's opium trade, but the defection of a local Taliban commander, Mullah Salaam, helped turn the tide in favour of Western forces.

The town was recaptured in December 2007 after fierce fighting between the Taliban and Nato and Afghan forces and became a vital strategic position.

UK forces led the counter-insurgency operations in Helmand during the protracted conflict with the Taliban and lost more than 400 soldiers there in the process.

Now the Taliban have recaptured the key parts of the province, in the first summer fighting season since foreign troops formally stepped back from combat roles in the country.

Violence has increased sharply across Afghanistan since most foreign forces withdrew in December, leaving only a small contingent of about 12,000 Nato troops to train Afghan forces.

The Taliban have extended their influence in Helmand into isolated towns in the north of the province, where Afghan security forces are vulnerable to attack.