At least three Afghan soldiers and more than a dozen Taliban militants were killed on Saturday in southern Afghanistan, the Afghan army said.

An Afghan army convoy was struck by a roadside bomb in Girishk district of troubled Helmand province killing three Afghan troopers, southern military corps commander General Shair Mohammad Zazai told AFP.

Separately, a joint raid by the Afghan army and international forces on a compound in Naw Bahar district, in neighbouring Zabul province, killed 18 Taliban fighters, he said.

The bodies of all 18 militants were recovered from the compound, along with weapons and vehicles, he said.

In Qalat, capital of southern Zabul province, Afghan and international forces killed one militant and captured three others, including a suspected "Taliban IED facilitator" who had taken refuge in an Afghan Red Crescent office, NATO said in a statement.

After a firefight in which the militant was killed, the joint force searched the compound and detained the militants, including the facilitator, NATO's International Security Assistance Force (ISAF) said.

IEDs, or improved explosive devices, have become the Taliban weapon of choice in the escalating insurgency, which is at its deadliest since the Islamist regime was overthrown in late 2001.

"This facilitator was responsible for financial support of militant activities and the transport of IED materials and weapons into the area," ISAF said.

Another operation, conducted overnight Friday by Afghan soldiers in Zabul's Mizan district, resulted in the deaths of four militants, Zazai said.

Southern Afghanistan has seen most of the insurgent activity in a war now in its ninth year, but the militants have in recent months extended their attacks to northern and western parts of the country.

There are around 100,000 foreign soldiers under NATO and US command in Afghanistan fighting alongside Afghan forces.