By THE ASSOCIATED PRESS

              BOGOTA, Colombia (AP) -- Setting aside animosity and ideological differences,
              Colombia's two largest guerrilla groups are increasingly joining forces on the
              battlefield.

              Some observers predict a short-lived alliance against rightist paramilitaries and
              counter-drug operations. But others said a frighteningly potent merger may be on the
              way between the Revolutionary Armed Forces of Colombia, or FARC, and the
              National Liberation Army, or ELN.

              In the latest evidence of cooperation, the army says 18 guerillas killed in a battle
              over the weekend were rebels from a joint-operation involving the two groups and a
              smaller insurgent faction.

              ``They were fighting alongside one another,'' said Gen. Martin Carreno of the army's
              5th Brigade in northern Colombia. ``That has to be a concern.''

              Colombia's 37-year war has been escalating as right-wing paramilitaries challenge
              guerrillas for territory, and the United States provides the armed forces with
              helicopters and Special Forces training under the Plan Colombia anti-drug scheme.
              At least 3,000 people are killed every year.

              Last week, top ELN commander Pablo Beltran said close ties between Colombia's
              army and paramilitary groups have left his group with no choice but to ally with the
              FARC at different strategic points throughout the countryside.

              One such area is southwestern Cauca State, where the two groups are reportedly
              teaming up against paramilitary fighters who killed up to 40 villagers in an Easter
              Week massacre.

              Some say these joint military offensives foretell a frightening change of heart for the
              rebel factions.

              It could end ``with the two groups joining together under one administrative and
              military front that will be very powerful and very dangerous,'' said Alejo Vargas, a
              history professor at Colombia's national university

              Gains by paramilitaries may not be the only reason for the newfound friendship
              between the 16,000-strong FARC and the 5,000-member ELN.

              Some experts speculate the move could help the FARC gain new territory in the
              face of U.S.-backed Plan Colombia, which has targeted some of its most important
              strongholds.

              ``The FARC is expecting that Plan Colombia's offenses are going to push them out
              of the south where the coca is grown,'' said Walter J. Broderick, an Australian who
              has written two books on the ELN. ``Now they want to concentrate their efforts in
              the rest of the country where the ELN is strongest.''

              For the ELN, teaming up with its once-bitter rival could mean the faction is too
              weak to continue fighting on its own.

              ``Their tank is empty,'' Alfredo Rangel, a former national security adviser, said.
              ``They want to get out with honor, they want to make peace. Until the government
              listens to them they will be ready to join with the FARC just to stay around.''

              Although both have been fighting for power since the 1960s, and each has turned to
              kidnapping and drug profits to sustain their operations, the FARC and the ELN are
              very different.

              Centered in the south, the FARC is officially Marxist, but its leaders are mainly
              aggrieved peasants who claim to be fighting for land reform and an end to
              corruption.

              The ELN began as a combination of radical university students and priests inspired
              by the Cuban revolution. The group opposes foreign oil exploitation and frequently
              bombs pipelines.

              The factions have generally avoided battles with one another. They have even
              worked together and staged a 1990 summit to announce they were forming a united
              front, which quickly collapsed.

              ``They don't like taking orders from each other very much,'' said Broderick. ``That
              will make any alliance they have now hard to maintain forever.''

              Their relationship has never been cozy. Last summer, ELN forces and the FARC
              clashed violently in the Cauca region where the two now claim to be working
              together.