Taliban militants attacked police checkpoints ringing a key provincial capital in southern Afghanistan for the second time in a week, sparking a battle in which 18 insurgents were killed, an official said Wednesday.

The late Tuesday attack came only two days after hundreds of militants gathered on the horizons of Lashkar Gah for an apparent large-scale assault on the capital of Helmand province. NATO called in fighter aircraft and 60 militants were reported killed.

The second attempt on the capital of the world's largest opium producing region would appear to signal the Taliban's interest in disrupting a major government center.

Large-scale Taliban attacks on major Afghan towns have been rare since the 2001 U.S.-led invasion, and the latest push on Lashkar Gah could be a worrying development for NATO and U.S.-led troops. Fighting typically takes place in small villages and rural areas.

Tuesday's battle killed 18 militants and wounded three police, said provincial police chief Assadullah Sherzad.

Sherzad said authorities recovered only one militant body and that the others were carried away by fighters. Afghan officials say they rely on intelligence reports to form militant death tolls, but government officials have been known to exaggerate such tolls in past battles.

Insurgency-related violence has killed more than 4,800 people — mostly militants — this year, according to an Associated Press count of figures from Western and Afghan officials.

In a separate incident, six policemen died after a shootout among officers inside a police checkpoint about 15 miles north of Lashkar Gah, said Daud Ahmadi, the spokesman for the provincial governor.

"We are investigating how and why this incident happened," Ahmadi said. He provided no other details.