At least 11 members of Afghanistan's security forces have been killed in a Taliban assault on government buildings in an unstable district in southern Helmand province, officials said on Thursday.

They said that four soldiers and seven police officers were killed when the insurgents overran Musa Qala — which has shifted between government and Taliban control for years — late on Wednesday.

Saqi Jan, police logistics expert in the district, said four soldiers and four police were killed in the attack. Earlier the spokesman for the provincial governor, Omar Zawaq, said another three policemen had been killed.

Jan said that police had regained control of the area around the district police headquarters after several hours of fighting. The Taliban claimed responsibility for the attack.

Afghan security forces are facing one of the biggest challenges of the 14-year insurgency, as they take on the Taliban without the backing of international combat forces, most of which withdrew last year.

In some regions, the Taliban have joined forces with other groups, notably the Islamic Movement of Uzbekistan and the East Turkistan Independence Movement.

The black flag of the Islamic State (ISIS( group has also been raised in some regions, notably in the north, but the group that now controls large parts of Iraq and Syria is not believed by officials to have an institutional foothold in Afghanistan.

The Taliban recently released an open letter to ISIS, telling its members to stay out of Afghanistan.

The Afghan insurgents have also released an appeal to supporters, coinciding with Ramadan — the holy month marked by fasting — that began across most of the Muslim world on Thursday. The statement on the group's website notes regular alms giving by Christians and Jews, and calls on Muslims to donate to its cause.

"Muslims, too, out of their true spirit of faith are not to forget their Muslim brothers defending and upholding the truth in Afghanistan and extend a helping hand in the form (of) financial aid," it says, providing an email addresses for contributions.