Leftist rebels killed at least 25 soldiers in separate clashes Saturday , the worst death toll in a single day for the military since President Alvaro Uribe came to power three years ago on pledges of crushing the guerrillas. Another 18 soldiers were reported missing.

Fighting broke out in southwestern Putumayo state when as many as 300 rebels of the Revolutionary Armed Forces of Colombia, or FARC, ambushed an army convoy during an attack targeting several nearby oil wells, said Gen. Carlos Lemus, inspector general of the army.

He said at least 19 soldiers were killed and 18 were missing during the battles near Puerto Asis, 330 miles southwest of Bogota. Putumayo is rich in oil and one of Colombia's main cocaine-producing centers.

Also Saturday, government troops launched an offensive to dislodge leftist rebels blocking a road in northeast Colombia, leaving at least six soldiers dead, local army commander Gen. Edgar Ceballos said.

Since the start of the year, the FARC has launched some of its boldest attacks on the military, killing more than 130 soldiers and shattering notions that the rebels have been brought to their knees.

The ferocity of the FARC attacks has led many observers to question Uribe's claims that the rebels have weakened since he ordered a costly, U.S.-backed military buildup upon coming to office in August 2002.