BOGOTA, Colombia (CNN) -- The Colombian government claimed
                  Wednesday that at least 50 leftist rebels were killed in an army attack on a
                  rebel stronghold in northwest Colombia.

                  The attack began Tuesday in La Llorona, a canyon in the foothills of the
                  Cordillera Occidental mountains where the FARC, Colombia's largest rebel
                  group, maintains a compound. Fighting continued Wednesday, and military
                  sources said more deaths were expected.

                  The fighting is along a highway linking Colombia's main banana-growing
                  region, Uraba, with the country's interior. FARC and right-wing paramilitary
                  groups have long battled for control of Uraba.

                  While an official statement issued in Bogota said at least 50 rebels died, a
                  commander in the area, Col. Diego Gutierrez, said 40 to 50. He said four
                  government soldiers had also been killed.

                  "The combat has been intense since" Tuesday, Gutierrez told local radio.
                  Troops backed by air force bombers and helicopter gunships were pitted
                  against a FARC force thought to be at least 700 strong, he said.

                  There was no independent confirmation of the number killed.

                  73 reportedly killed near Cartagena

                  Tuesday, the government's human rights ombudsman said 73 people had
                  been killed during a week of clashes in a rural area south of Cartagena,
                  Colombia's leading Caribbean resort city.

                  The fighting there involved FARC and a right-wing paramilitary  group.
                  Sixteen civilians were killed, along with 26 FARC rebels and 31
                  paramilitaries, according to the ombudsman's office.

                   Military sources would confirm only that 16 civilians had died.

                  Government steps up war

                  The latest attacks indicate the government may be stepping up its war
                  against rebels, who have been battling the government for more than three
                  decades and have de facto control over about 40 percent of the country.

                  Peace talks between rebel groups and the government, begun earlier this
                  year, stalled almost immediately. And the peace process was dealt a serious
                  blow with last week's revelation that FARC was behind the recent slayings
                  of three U.S. humanitarian workers, whose bullet-riddled bodies were found
                  just over the Venezuelan border.

                  The new offensive against the rebels has the backing of the United States
                  and Colombia's neighbors. Peru and Ecuador have sent more troops to their
                  northern borders under a U.S.-devised plan to contain drug traffickers. Both
                  U.S. and Colombian officials say the rebels are involved in the drug trade.

                  Throughout the region, U.S. military teams are training armies. More than
                  470 American servicemen are in Ecuador, Venezuela and Peru, according to
                  Steve Lucas, a spokesman for the U.S. Southern Command.

                  This month, Colombia began to overhaul its ill-equipped and poorly
                  motivated military and create a more professional force that could move
                  more decisively against the rebels should the peace process break down
                  irretrievably.

                  Political observers believe the possibility of direct U.S. intervention in
                  Colombia's conflict is remote. But some observers are troubled by
                  Washington's ever-greater behind-the-scenes role.

                  "U.S. intervention is based on the deadly equation that they give the military
                  technology and the weapons and we provide the dead," said Alejandro
                  Santos, a columnist in Semana, a leading news magazine.

                                    Reuters contributed to this report.