The Taliban captured a key district centre in Afghanistan's southern Helmand province on Thursday while in the country's north, an officer turned his rifle on sleeping colleagues, killing nine policemen in the latest "insider attack".

Helmand, which accounts for the bulk of Afghanistan's billion dollar opium crop, is already largely in the hands of the Taliban but the capture of Sangin - where US and British forces once suffered heavy casualties - underlines its growing strength in the south.

The district's police chief, Mohammad Rasoul, said the Taliban overran Sangin centre early Thursday.

Taliban spokesman Qari Yousuf Ahmadi issued a statement claiming the Taliban capture of the district. He said the area had been bombarded by foreign forces following the withdrawal of Afghan troops and police, causing heavy damage to buildings and infrastructure.

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Rasoul told AP news agency that the district headquarters had been poorly protected and at the time of the Taliban siege, only eight policemen and 30 Afghan soldiers were on duty.

Afghan security forces were now amassing nearby for a full-scale counterattack in a bid to retake Sangin, Rasoul added.

"We are preparing our reinforcements to recapture the district," Rasoul said.

NATO spokesman William Salvin said in a statement that Afghan troops remained in Sangin district but had relocated several kilometres outside the district centre.

In Kabul, a politician from Sangin, Mohammad Hashim Alokzai, urged the military to move quickly to retake the district, saying its fall could have devastating consequences for Helmand, where the provincial capital of Lashkar Gah has in the past months also come under constant and heavy attack by the Taliban.

"The seizure of Sangin is a major tactical triumph for the Taliban," Michael Kugelman, senior associate for South Asia at the US-based Wilson Centre, said on Thursday. The group "has taken over a major urban space in one of its major stronghold provinces, amplifying the major threat that the group poses to Afghanistan nearly 16 years after it was removed from power".

"It's hard to overestimate the significance of Helmand - it's strategically located near Pakistan, it's a bastion of the opium trade," said Kugelman.

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In northern Kunduz province, police spokesman Mafuz Akbari said the insider attack on Thursday that claimed the lives on nine policemen took place at a security post and the assailant escaped.

Afghanistan has seen a spike in so-called insider attacks. In such incidents, attackers who turn their rifles and kill colleagues usually end up stealing their weapons and fleeing the scene to join rebel groups.

Akbari said the assailant had gone over to the Taliban. He said the attacker and the Taliban gathered the bodies of the dead policemen and set them on fire.

Taliban spokesman Zabiullah Mujahid claimed responsibility for the attack, but denied a policeman had been involved, or that the Taliban burned bodies.

The conflicting accounts could not be immediately reconciled. The region is remote and not accessible to reporters.

Afghan forces have come under intensified pressure by insurgents in both Helmand and Kunduz.

According to US estimates, government forces control less than 60 percent of Afghanistan, with almost half the country either contested or under the control of the Taliban, which is seeking to re-impose Islamic law after their 2001 ouster.