Purported Taliban spokesman Qari Yousef Ahmadi said the group's leaders announced the beginning of operation "Kamin," or "Ambush."

"In this operation, we will target our enemies and use our tactics suicide bombs, remote-controlled (roadside bombs) and ambushes against occupying forces and the government," Ahmadi said by satellite phone from an undisclosed location. "We start this operation today in all of Afghanistan."

After a winter lull in violence, militant attacks and military operations have surged. Nato and the US-led coalition stepped up operations in the early spring, hoping to pre-empt a spring offensive by militants that threatened the already-shaky grip of President Hamid Karzai's government.

In Kandahar, the Taliban ambushed a police convoy on Saturday, and the ensuing one-hour gun battle killed two policemen and wounded three others, said Shah Wali Kot district chief Obaidullah Khan. He said the Taliban also suffered casualties, but he had no details.

In neighbouring Zabul province, a roadside bomb exploded Saturday as an Afghan army vehicle passed, wounding two soldiers, said Gen. Rahmatullah Raufi, the regional army corps commander.