BOGOTA, Colombia (CNN) -- Colombian rebels are threatening
                      to kill four U.S. citizens if they prove to have ties to U.S.
                      intelligence agencies, while the country's second-largest rebel
                      group said Sunday that it is serious about peace talks.

                      A jailed leader of the National Liberation Army (ELN) said the
                      group was serious about an accord it signed a month ago in Spain
                      that would lead to peace talks in June.

                      "We all have a historic responsibility to join together to search for a
                      way out of this common grave," said Felipe Torres in an interview
                      with the El Espectador newspaper in Bogota.

                      But the largest and oldest rebel group, the Revolution Armed
                      Forces of Colombia, or FARC, has refused to participate in the
                      talks.

                      The group kidnapped four U.S. citizens and an Italian last Monday
                      at a FARC roadblock near Bogota.

                      A FARC commander, known by his alias of "Romana," told
                      reporters over the weekend that he would kill the Americans if
                      they proved to be U.S. undercover agents. If they don't have ties
                      to U.S. intelligence, Romana said they would only be released in
                      exchange for a large ransom.

                      Colombian media has identified the four as Thomas Fiore, Peter
                      Chen, Todd Mark and Louise Agustine. The Italian was identified
                      as Vito Candela.

                      The U.S. State Department said the four Americans were on a
                      bird-watching expedition. Colombia has one of the world's most
                      diverse bird populations, which attracts visitors worldwide.

                      The rebels have been fighting their insurgency for more than 30
                      years. The U.S. State Department has warned visitors to travel to
                      Colombia only if "strictly necessary."

                      The Colombian government said the warning was unnecessary.

                      On Friday, Romana stepped out of hiding when he released nine
                      Colombian hostages to the Red Cross at a news conference in El
                      Calvario, 50 kilometers (31 miles) east of Bogota. Romana has a
                      $75,000 price on his head by the Colombian government.

                      The rebel's growing military presence is raising concerns in the
                      United States. The new U.S. ambassador, Curtis Kamman, has
                      said the rebels are one of the greatest threats to Colombia's
                      democracy and that he will push for more military aid.

                      The United States has 200 advisers in Colombia, ostensibly for
                      anti-drug efforts. FARC said two weeks ago that it would target
                      U.S. advisers, accusing them of running covert counter-insurgency
                      operations.

                      However, the ELN's Torres said Sunday that if the United States
                      becomes more involved in the civil conflict, it would turn into
                      another Vietnam.

                      "If the United States gets involved, then there will be a generalized
                      action," he said. "(Colombia's) insurgent groups will become true
                      armies and this will become another Vietnam."

                      FARC and the ELN were both formed in the mid-1960s and have
                      a combined combat force believed to number about 15,000.

                      The Associated Press and Reuters contributed to this report.