Before this month, Colombia's FARC rebels had been virtually dormant for two years. But the third major battle with the army in two weeks left at least 28 people dead.

By STEVEN DUDLEY

BOGOTA - Seventeen soldiers and 11 guerrillas were killed and eight soldiers were missing in the third major clash over the past two weeks, this time in a mountainous region in northwestern Colombia.

Colombian army chief Gen. Carlos Ospina said the combat began Tuesday when the military killed 11 members of the Revolutionary Armed Forces of Colombia, or FARC, in Mutatá, about 175 miles northwest of Bogotá.

The FARC, Ospina said, regrouped and ambushed the soldiers as they returned to base, leaving at least 17 dead. Eight soldiers are still missing.

This was the third significant rebel attack on the military in the past two weeks. Before this month, the FARC had been virtually dormant for two years, while the government of conservative President Alvaro Uribe executed a massive offensive involving 15,000 soldiers. Dubbed ''Plan Patriot,'' the military has sought to regain control over traditional rebel strongholds in the southeastern part of the country, with some success.

On its website, the Colombian Defense Ministry claims it has killed more than 4,500 guerrillas and captured another 15,000 from both the FARC and the smaller National Liberation Army since Uribe took power in August 2002. Officials also significantly lowered the estimates of the FARC's strength from 19,000 to 13,000.

Military analysts strongly dispute these figures and question government claims that the FARC, which has been fighting the government since 1964, may be on the ropes.

In recent weeks, the rebels killed 15 Marines when they overran a navy base along the southwestern coast and killed eight soldiers with a roadside bomb in the central part of the country. On Wednesday, they were reported to be surrounding several villages near the western coast.

''I think there are signs that they've been given the green light'' to attack, said Sergio Jaramillo, the director of the Colombian think tank Ideas for Peace. ``When the FARC sees an opportunity, they're going to take it, especially when it relates to drugs.''

The southwestern province of Nariño has been especially hard hit by the recent violence. The navy base attacked was in Nariño, and last week the guerrillas burned several buses around the province. Nariño reportedly has Colombia's highest number of coca and opium poppy fields. It's also an important corridor for shipping drugs and guns through Colombia's southern Pacific coast and land border with Ecuador.

The attacks illustrate just how difficult it is to control Colombia, which is roughly three times the size of Iraq.

The area where this week's battle occurred, near the border with Panama, has been in dispute for years between the FARC, the government and right-wing paramilitaries that often work in conjunction with government units.

The U.S. government has provided Colombia with close to $3 billion since 2000, most of it in aid for the military, to fight the armed groups and control the flow of drugs to the United States. The Bush administration has proposed sending another $600 million this year to help it.