Clashes and an airstrike in southern Afghanistan killed 13 suspected Taliban militants and three police, officials said Monday.

The police officers died Monday when militants attacked a checkpoint on the road linking the southern town of Kandahar with Spin Boldak on the Pakistan-Afghanistan border, said Esmatullah Alizai, Kandahar province's police chief.

A NATO airstrike, meanwhile, targeted a compound housing Taliban militants in Shahjoy district of Zabul province on Sunday, killing seven suspected militants inside, said Ubaidallah Khan, the district's police chief.

Also Sunday, NATO-led troops and police clashed with suspected Taliban militants in Kandahar's Zhari district, leaving six militants dead, Alizai said.

Police recovered the militants' bodies and their weapons, he said. There were no NATO or police casualties in that clash.

Afghanistan's south is the center of the Taliban insurgency. Last month, NATO-led troops launched their biggest offensive yet in the region aimed at winning over the local population and targeting militants and their supply routes.

Afghan and NATO officials say they expect violence to increase this spring and summer. Last year, Taliban militants set off a record number of suicide and roadside bombs.

Afghan intelligence officers, meanwhile, detained three Pakistanis and one Afghan man in eastern Nangarhar province, alleging they were preparing suicide attacks inside Afghanistan, an intelligence officer said on Monday, on customary condition of anonymity.

The four were carrying 130 kilograms (290 pounds) of explosives, 12 fuses and an AK-47 assault rifle, and were detained on Thursday, shortly after crossing into Afghanistan from Pakistan, official said.