BY MARIKA LYNCH AND TIM JOHNSON

  BOGOTA - For the first time, Colombian guerrillas Saturday acknowledged seizing three U.S. government contractors after their plane went down in the southern
  mountains on Feb. 13.

  In response to the kidnapping, scores of U.S. troops have poured into the South American nation, bringing the number to record levels and drawing the United States further into Colombia's prolonged civil conflict.

  In a communiqué, leaders of the Revolutionary Armed Forces of Colombia, or FARC, said they could guarantee the safety of the three men only if the Colombian military stopped running patrols in a southern region considered rebel territory. Complying, however, would mean calling off the massive search for the men whose small Cessna plunged into a mountainous jungle nearly two weeks ago.

  President Alvaro Uribe rejected the deal Saturday, and the notion that the government would bow to rebel demands.

  ''Operations are managed by the [Colombian army] . . . not the FARC,'' Uribe said.

  Some U.S. sources familiar with U.S. operations in Colombia privately expressed fears that the men might have been killed already by an elite rebel unit called the Teofilo Forero Brigade, which apparently seized the hostages. One defense industry source said the fears were based on communications overheard by U.S. intelligence indicating that the Forero unit had received permission to carry out ``executions.''

  The information, however, could not be confirmed.

  AMERICANS TARGETED

  Opposed to increasing U.S. involvement in Colombia, the rebels have said Americans are targets in their war against the government. Despite a request from U.S. officials, the FARC gave no proof Saturday that the three Americans were alive.

  The rebels killed two others aboard the plane -- a Colombian intelligence sergeant and an American -- as they apparently tried to escape shortly after the crash, 200 miles south of Bogotá, authorities said.

  Also Saturday, new obstacles in the search for the men loomed. Search teams operating out of three air bases get supplies -- food and fuel -- brought over land routes frequently interrupted by guerrilla road blocks.

  ''There's not enough fuel to keep operations running for more than the next three days,'' said a U.S. source who is closely monitoring rescue efforts. He also voiced
  frustration at what he described as poor coordination between the Colombian and U.S. forces.

  DETAIL CONFLICTS

  The FARC said the three Americans were CIA operatives gathering information on the rebels, and that the plane crashed because one of their units shot it down -- two assertions U.S. officials have denied.

  While the plane was riddled with bullets, both Colombian and American authorities have insisted the Cessna made an emergency landing because of engine trouble.

  U.S. authorities also have said the men are defense contractors working for the U.S. Embassy, not the CIA.

  Officials have not expanded on their mission, but the men's plane, a Cessna 208, carried electronic intelligence gathering equipment, defense industry sources said. The contractors work for California Microwave Systems Co., according to congressional and defense industry sources.

  The company, a subsidiary of Northrop Grumman, specializes in airborne reconnaissance and surveillance equipment, according to its website, which also says the
  company has contracts with the U.S. military and international defense organizations.

  Spokesmen for both Northrop Grumman and California Microwave Systems said their companies had ''no comment'' about the incident, and referred calls to the U.S. Embassy in Bogotá.

  So far the rebels have not asked for a ransom, or offered to exchange their hostages for imprisoned FARC members, as analysts had envisioned.

  Instead, their sole demand was worded this way: ``We can only guarantee the life and physical integrity of the three gringo officials in our control if the Colombian army immediately suspends military operations and overflights in the area.''

  Asked about the demand, White House spokesman Ari Fleischer said only that it was a reminder of how dangerous the group known as the FARC is, ``how they are well-known for hostage-taking and for trying to bring harm to the civilian population of Colombia.

  The kidnapping -- the first involving U.S. government employees in decades -- comes at a time when the U.S. is expanding its involvement in Colombia. Last year, the Bush administration decided that aid could be used not only to fight drug traffickers, but also to battle leftist guerrillas. Recently, 70 U.S. Green Berets were sent to train Colombians to protect an oil pipeline frequently targeted by rebels.

  Now, the number of U.S. military troops tops 400. As recently as Jan. 13, there were only 208 military personnel in Colombia, according to a congressional report.

  SAFETY CONCERNS

  Meanwhile, a former U.S. lawmaker and CIA analyst raised questions Saturday about the safety of all contractors in Colombia.

  Bob Barr, a four-term conservative congressman from Georgia who is now a lobbyist, visited Colombia last fall and issued a scathing report on operations under the $2 billion aid package known as Plan Colombia.

  The aid package provides money to fight Colombia's drug industry, which supplies 90 percent of the United States' cocaine. The FARC earns money off taxes from coca growers.

  In the past five years, 12 Americans -- including six government contractors -- have died in Colombia because of lax safety measures, Barr said in his report.

  A ''little shot of reality here: Today in Colombia, the FARC has a bounty on the heads of Americans working with the Colombian National Police and military,'' he said. `They are all targets.''

  Marika Lynch reported from Bogotá, and Tim Johnson reported from Washington.