Taliban fighters in southern Afghanistan attacked a checkpoint with firearms and hand grenades, killing 11 policemen and confiscating weapons and ammunition.

But a provincial official said Tuesday's attack in Helmand province's capital Lashkar Gah could be an "insider attack" as one of the guards was still missing.

"An investigation is ongoing to find out if someone from inside has defected to the Taliban and paved the way for this crime," he said.

Taliban spokesman Zabihullah Mujahid told Al Jazeera the group was responsible for the attack, saying it killed 11 policemen and "claimed the area and weapons".

Din Mohammad, a doctor at the Lashkar Gah Hospital, confirmed the death toll.

In a separate incident, another 12 policemen were killed in Marjah district of Helmand after an hour-long gun battle, Mujahid said.

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The attacks underlined the threat facing Afghan security forces in the opium-producing province, where they struggle to match well-equipped Taliban fighters who now control most of Helmand, including areas in Lashkar Gah.

Earlier this month, an Afghan policeman turned his rifle on his colleagues in northern Faryab province, killing eight policemen as they slept in an outpost in the district of Almar. He then collected all their firearms and fled the scene.

British and US forces suffered their heaviest casualties of the war in the province in years of fighting following the removal of the Taliban in 2001.

Afghan security forces now control less than 60 percent of the country, according to US estimates, with the Taliban holding about 10 percent and the remainder contested between various armed groups.