A joint Afghan and U.S.-led coalition operation against insurgents in southern Afghanistan killed more than 20 Taliban fighters Sunday, while a militant ambush in the northwest killed four policemen, officials said.

The joint operation in Zabul province included ground forces and airstrikes. The battle killed more than 20 Taliban, said provincial police chief Abdul Rahman Sarjung. After the operation, a roadside bomb exploded and killed one Afghan policeman as the forces were returning to base, he said.

Elsewhere, militants attacked a police security post in northwestern Faryab province, sparking a one-hour battle that killed four police, said Khalil Andrabi, the provincial police chief.

Another Taliban attack in the eastern province of Paktika killed the police chief in Sarhawza district, while militants elsewhere in Paktika ambushed a truck of private security guards, killing four of them, said Hamidullah Zhwak, a spokesman for the governor.

Militants have stepped attacks in the last several years and now control many rural areas in Afghanistan. President Barack Obama is sending 21,000 additional troops this year to help increase security in regions where the Afghan government has little control.