Nine people - seven of them children - were wounded when leftist FARC rebels launched a grenade at a navy checkpoint in the Pacific port city of Buenaventura, the Colombian military said Monday.

Members of the Revolutionary Armed Forces of Colombia, or FARC, were behind Sunday night's attack, the commander of the marines' 2nd River Brigade, Col. Gonzalo Aladino Franco, told reporters.

"Apparently, the subjects launched the explosive device from an area of shallow water, behind some homes, and they fled," he said.

Wounded in the blast were a marine, a civilian woman and seven children who happened to be playing on a nearby vacant lot, the navy said.

The injured were treated at a hospital in Buenaventura.

The navy will bring a formal criminal complaint against the FARC for violations of international humanitarian law, Aladino said.

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