Eight policemen were killed by their colleagues after they were poisoned in their base in southern Afghanistan in the latest "insider attack".

The Taliban claimed responsibility for the incident, which happened in Nawshar district of southern Zabul province late on Friday, as the group escalates a deadly winter campaign of violence.

"The infiltrators first poisoned their colleagues and then shot them dead," provincial spokesman Gul Islam Seyal told AFP news agency on Saturday, adding the attackers fled the area taking all the weapons and munitions from the base.

The governor of Zabul, Bismillah Afghanmal, said they had launched an investigation into the incident.

Taliban spokesman Zabihullah Mujahid said in a message to the media that the group's "infiltrators" carried out the attack.

The Reuters news agency, quoting local officials, said the attackers defected to Taliban.

So-called insider attacks - when Afghan soldiers and police turn their guns on their colleagues or on international troops - have been a major problem during the more than 15-year-long war.

Such attacks have sapped morale and caused deep mistrust within security ranks.

In a similar incident last month, an Afghan policeman linked to the Taliban shot dead 11 of his colleagues at a checkpoint in the neighbouring Helmand province.

And last September, two Afghan soldiers with suspected Taliban links killed at least 12 of their comrades as they slept in the volatile northern province of Kunduz.

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Separately on Saturday, gunmen attacked a military airbase in the eastern province of Khost.

Faizullah Ghairat, security chief of Khost province, said three assailants attacked the base, close to the border with Pakistan. One was killed while two others escaped, he said.

The attackers tried to enter the base but were prevented by guards.

"Our forces have shot one armed attacker dead, and at least two [would be] suicide attackers are at large, we are looking for them," he said.

Nine people, including seven CIA agents, were killed in a suicide attack carried by a Jordanian associated with al-Qaeda on the same military base in December 2009. Camp Chapman in Khost is said to be instrumental in providing intelligence to the CIA for drone attacks across the border in Pakistan.

There was no immediate comment from the headquarters of the NATO-led Resolute Support mission in Kabul.

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The incident comes just before the normal start of the spring fighting season, when the warmer weather brings increased operations by both rebel and government forces.

Afghan and US officials have warned that Afghanistan will see increased fighting this year as the Taliban steps up a campaign that has cut the area controlled by the government to below 60 percent.

Earlier this week, the head of US Central Command, General Joseph Votel, asked for more American troops to join the roughly 8,400 already stationed there.

The Afghan interior ministry said over the past 25 hours, security forces had killed 51 fighters in counterterrorism operations across Afghanistan.