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At least 12 Afghan policemen have been killed after an "undercover" Taliban militant shot his colleagues dead at a checkpoint in Helmand province on Monday.

Silenced weapons and hand grenades were used in the attack, which took place as the officers slept in their beds at the barracks.

The attacker, or group, made off with ammunition and weapons. One guard from the division is still missing leading police to believe the killer, or facilitator, was an undercover militant.

Officials described the killings as an "insider attack" while the Taliban have since claimed responsibility.

"A policeman affiliated to the Taliban shot 11 of his colleagues, killing all of them," an anonymous provincial official told AFP.

"He then fled the area, taking all the ammunition and firearms with him."

An Afghan police chief said that the incident could have involved a group of Taliban, while Reuters put the death toll at 12.

"The Taliban attacked a guard with silenced guns and then entered the check post," Helmand deputy police chief Haji Gulai told reporters.

"They attacked other policemen with hand grenades and killed all of them. They later took their weapons and ammunition and escaped." he said.

The incident occurred late Monday in the southern Afghan provincial capital Lashkar Gah, an area known for its strong Taliban presence and opium-production.

They attacked other policemen with hand grenades and killed all of them. They later took their weapons and ammunition and escaped. - Haji Gulai, Helmand deputy police chief

Police are hunting the killer or killers. Many of the dead appear to have been shot at close range, Shir Mohammad, a police officer stationed at a nearby outpost, told AFP.

Taliban insider attacks on Afghan security forces or international coalition troops have been common during the 15-year-long war.

The latest incident comes as insurgents step up attacks on security forces in Afghanistan, leading to fears that the government could lose control of the country.

Meanwhile, 12 policemen were killed in an hour long shoot out with Taliban in Helmand province on Monday.

The precarious security situation has forced NATO to send in reinforcements, after years of reducing their presence in Afghanistan.

Afghan security forces have lost control of several major cities and towns to the Taliban in the past few months.

Most of these areas have been recovered by the Afghan army, but not without international air support and sustaining heavy losses.

Meanwhile, one study has shown that the Afghan government controls just 60 percent of the country, according to Reuters.

Kabul is also facing mass desertions from its armed forces, while the Afghan population's faith in the central government in providing security is being seriously stretched.

Around 11,500 Afghan civilians were killed or wounded in 2016, around a third children, while 590 casualties were sustained from air strikes

Agencies contributed to this story.