BOGOTA, COLOMBIA, JAN. 2 -- Marxist guerrillas killed 12 policemen and damaged an important bridge today, cutting off traffic from a major holiday resort area, as one of their largest offensives against the government continued.

The attacks, part of the rebels' "Operation Wasp," came a day after the guerrillas carried out raids across the country that killed 13 policemen, two security agents and an army lieutenant, and damaged a key oil pipeline.

While international attention on Colombia has focused on the war against drug traffickers, the government also is battling the longest-running Marxist-led insurgency in Latin America, led since the 1950s by two hard-line groups -- the 6,000-man Revolutionary Armed Forces of Colombia (FARC) and the National Liberation Army (ELN), with an estimated 4,500 adherents.

The two groups, along with several smaller organizations, maintain a loose alliance known as the Simon Bolivar Guerrilla Coordinator.

The alliance vowed to step up the war following the Dec. 9 attack by the army and air force on the strategic FARC headquarters known as Casa Verde (Green House), driving the rebel leadership out of the area for the first time in two decades and sharply escalating the war on the insurgents after a year of relative tranquility on that front.

Government and diplomatic sources say that with terrorism from the Medellin cocaine cartel declining -- after it dominated the military agenda for 16 months -- the armed forces are anxious to return to fighting the guerrillas.

The FARC, which has held occasional, fruitless peace talks with the government for a decade, said the attack on Casa Verde signaled the end of the peace process. That group has been responsible for most of the recent wave of attacks.

Presidential adviser Jesus Bejarano said at a press conference today that "we will continue, despite all the difficulties, the same disposition to talk. But the government will remain firm in not making concessions to acts of force or violence. On that basis, it is impossible to initiate any process of dialogue."

Col. Jaime Pena, a police spokesman in Bogota, said that at about 6:30 a.m. the rebels ambushed a truck carrying troops of the anti-narcotics police on a bridge near Cienaga, on the main highway that connects the tourist center of Santa Marta, on the Caribbean coast, with the rest of the nation.

The attack and ensuing fight closed the highway for seven hours and thousands of people trying to return from their holiday vacations were left stranded, according to radio reports from the scene.

"This is part of the terrorist strategy, which is designed to draw attention," Pena said. "While they talk of peace, they carry out acts of war."

On Tuesday, rebels damaged the main oil pipeline in the south, causing the loss of several thousand barrels of petroleum, officials said. Guerrillas of the ELN shot up the pipeline in two places, and then ambushed police sent to investigate, killing five. The attacks took place as the government was making some progress in bringing armed groups into the political process.

Nine months ago, the M-19 rebel group, responsible for some of the most notorious terrorist attacks in the 1980s, signed a peace agreement with the government and formed a legal political party. In exchange for giving up the armed struggle, they were granted blanket amnesty for their guerrilla actions.

The party has grown rapidly, running third in presidential elections in May and winning the largest bloc of delegates in voting last month for a constituent assembly.

Similar peace talks with the government are currently under way with the Popular Liberation Army, one of the largest guerrilla groups; the much smaller Quintin Lame group, a militant indigenous movement; and the Revolutionary Workers' Party. The groups, while still armed, are honoring a cease-fire and are concentrated in camps. If they sign a final peace agreement with the government by February, they will be granted two assembly seats.