An explosion ripped through a line of trucks ferrying fuel to NATO troops in Afghanistan, setting several oil tankers ablaze Sunday at a backed-up Pakistani border crossing, police said.

The blast appeared to be the second terrorist attack Sunday in Pakistan and the second in a week to target a border crossing. Also Sunday, a suicide bomber targeted a police station in the northwestern Swat Valley, killing 15 cadets.

Local police chief Hasan Sardar said flames and smoke were billowing into the sky as authorities struggled to control the blaze Sunday night near the Chaman border crossing in Baluchistan province in Pakistan's southwest.

"It was a big explosion under one of the oil tankers that caused other vehicles to catch fire. The fire is spreading," Sardar told The Associated Press by phone.

"We are at the moment trying our best to control the blaze. We are not sure whether there is any human loss," he said. "It is just panic everywhere there."

The explosion Sunday night set at least three oil tankers, two container trucks and two dump trucks on fire, police officer Abdul Rauf said.

Chaman is one of two main crossing points for supplies for American and NATO troops fighting the Taliban in Afghanistan. The foreign troops get about 75 percent of their supplies through Pakistan.

The crossing has been closed for two days amid a dispute between Afghan and Pakistani customs officers that Rauf said had left more than 1,000 trucks backed up along the road to the border.

Another suicide bombing killed at least 19 guards further north at the Torkham checkpoint, the other main crossing and gateway to the famed Khyber Pass.

The Pakistani Taliban have vowed revenge after the loss of key territory and the death of their top leader, Baitullah Mehsud, in a CIA missile strike Aug. 5 further west near the Afghan border.