Four people were killed early Wednesday when leftist FARC rebels shot at a bus traveling in the southwestern province of Nariño, Colombian authorities said.

Fighters from the Revolutionary Armed Forces of Colombia, or FARC, attacked the vehicle on a road that runs along the Pacific coast, the provincial police commander, Col. Hugo Diaz, told Efe.

The bus, which was en route from Barbacoas to Pasto, Nariño's capital, was met with "bursts of rifle fire" around 2:00 a.m., he said.

Two men and two women - one of them pregnant - died and seven other passengers were wounded, Diaz said.

The most seriously injured passengers were taken to a hospital in Pasto.

"There were no battles at that site, nor security forces to attack, so it is a demented direct action against a bus that carried 33 civilians," the colonel said.

After shooting up the bus, the guerrillas parked another vehicle astride the road and slashed its tires.

Police and army troops towed away the vehicle in time to re-open the road by 6:00 a.m., Diaz said.

The assault on the bus came about 12 hours after a roadside bomb exploded in Popayan, capital of the southwestern province of Cauca, leaving a civilian dead and seven police wounded.

Earlier Tuesday, FARC guerrillas destroyed a portion of a bridge linking Popayan and Cali, the capital of neighboring Valle del Cauca province, shutting down a stretch of the Pan American Highway for several hours.

The FARC has battled a succession of Colombian governments since the mid-1960s. Thought to number nearly 20,000 fighters a decade ago, the Andean nation's largest insurgency has an estimated troop strength today of around 8,000. EFE