At least two dozen Afghan security forces have been killed in an insider attack on their base in southern Afghanistan, two officials told AFP news agency.

Rahmatullah Yarmal, spokesman for the governor of Zabul province, said in Friday's pre-dawn raid, six policemen opened fire on sleeping troops at a joint police and army headquarters near Qalat, the capital of the southern province, long considered a Taliban stronghold.

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"At 3am (10:30 GMT), 24 Afghan police and army forces were martyred when six police infiltrators opened fire on them while they were asleep," he said.

According to TOLO News, an Afghan news website, Ata Jan Haq Bayan, the Zabul provincial council chief, put the number of dead security personnel at 20 and the number of attackers at eight. He added that four other Afghan security forces were missing.

"The attackers had connections with the Taliban insurgents," Bayan said.

They fled in two military Humvee vehicles, along with a pickup truck, weapons and ammunition, AFP reported.

Councillor Asadullah Kakar also told the dpa news agency that eight insiders, with the support of the Taliban fighters, killed their comrades while they were sleeping.

The attackers have taken all weapons and ammunition, he added.

'Active defence'

The attack came a day after Asadullah Kalid, Afghanistan's acting defence minister, said the country's forces would switch to an "active defence posture" as the Taliban continued to attack even after the armed group signed a deal with the United States last month in the Qatari capital, Doha.

"The Taliban continued high level of violence despite the peace agreement," Kalid said in a video statement.

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"An active defence posture will reduce the restrictions on ANDSF (Afghan National Defense and Security Forces) and it will allow them to carry out operations against the Taliban plotting attacks against ANDSF," he added.

The release of thousands of Taliban prisoners agreed upon in the US-Taliban agreement has been delayed over Kabul refusing to free them all at once.

The United States in February signed a deal with the Taliban aimed at paving the way for them to negotiate with the Afghan government, including an agreement on withdrawing foreign troops.

The Taliban say it has held back from attacking international forces since then but has continued to attack Afghan forces, with US and Afghan officials calling for a reduction in violence.