A US military helicopter has crashed during a firefight with insurgents in a remote area of southern Afghanistan, killing seven Americans and four Afghans in one of the deadliest air disasters of a war now into its second decade.

The Taliban claimed they gunned down the Black Hawk.

American service personnel are dying at a rate of about one per day so far this year despite a drawdown of troops and dwindling attention on the home front.

That death rate has risen recently with the summer fighting season in full gear and a rash of attacks by Afghan security forces on their foreign trainers and partners.

NATO forces said they could not confirm what caused Thursday’s crash and stressed that it was still being investigated.

The Black Hawk was operating in support of an ongoing assault on the ground but initial indications were that it was not shot down, according to US officials.

Taliban spokesman Qari Yousef Ahmadi said insurgent fighters struck the helicopter in Kandahar province on Thursday morning. He declined to give further details in a phone call with The Associated Press.

The Kandahar provincial government backed the Taliban claim. It said the helicopter was shot down in Shah Wali Kot district, a rural area north of Kandahar city where insurgents move freely and regularly launch attacks. Provincial spokesman Ahmad Jawed Faisal did not provide details or say how the province had confirmed the information.

Thursday’s crash came less than a week after six American service members were gunned down, apparently by two members of the Afghan security forces they were training to take over the fight against the insurgency.

White House spokesman Jay Carney said on Thursday that the US is prioritising efforts to prevent more of these types of attacks.

The international force “is continually assessing and refining procedures in force protection so that we can both meet mission requirements and ensure the safety of our forces”, Carney said.

The Taliban said on Thursday the insider attacks are part of a strategy to undercut the alliance between the Afghan government and international forces.

Mujahideen have cleverly infiltrated the ranks of the enemy according to the plan given to them last year,” the militants said in their annual statement ahead of the Eid al-Fitr holiday marking the end of Ramadan, the Muslim holy month.