A rebel attack on a Navy base that left 15 dead and about 25 injured was the deadliest since the government launched a major offensive two years ago.

BY STEVEN DUDLEY

BOGOTA - A leftist guerrilla raid on a remote Colombian Navy base that killed 15 and wounded about 25 Tuesday was the first major rebel attack since President Alvaro Uribe launched a powerful military offensive two years ago.

Helicopter gunships, patrol boats and the heavily armed, slow-flying aircraft known as ''ghost'' planes were chasing the 200 members of the Revolutionary Armed Forces of Colombia, or FARC, who staged the attack, Navy officers said.

They said the rebels fired devastatingly powerful homemade mortar-bombs -- fashioned from cooking gas containers like those in barbecues -- at a Navy river base in the western Iscuandé section of Nariño province, about 350 miles southwest of Bogotá.

Among the dead were several Marines and so-called ''peasant soldiers,'' troops who serve in specific rural areas, usually where they grew up.

The attack was the FARC's largest and deadliest since 2002, when the hard-line Uribe ordered his military to launch Plan Patriota, an unprecedented offensive against FARC targets in southeastern Colombia involving 15,000 troops.

The military has boasted that the operation so far was a huge success, reporting more than 200 rebel deaths, 400 captures and thousands of desertions. But the latest attack showed the FARC still retains some punch.

''They're trying to show that Plan Patriota has not hurt them, that they can mobilize large numbers of men and hit hard when they want to,'' said Jaime Zuluaga, a political science professor at the National University in Bogotá.

A Navy communiqué said the attack Tuesday was the work of the FARC's 29th Front, which operates around Nariño, a mountainous coca and opium poppy-producing province that also serves as an important corridor for weapons and drug shipments via Ecuador and the Pacific.

The FARC, which has been fighting the government for four decades, is believed to have between 15,000 and 20,000 fighters. It finances itself partly through kidnappings, extortions and some participation in the drug trafficking that flourishes in this country of 45 million people. The U.S. State Department lists it as a terrorist group.

The smaller leftist National Liberation Army guerrillas and right-wing paramilitary groups created illegally to fight the leftist rebels are also active in Nariño.

Nariño grows more coca and opium poppy than any other province in the country, according to the latest statistics from the government's National Drug-Control Center.

The U.S. government has provided close to $3 billion in mostly military aid and training to Colombia since 2000 to help the government fight the drug traffickers and rebels.