BY TIM JOHNSON

  WASHINGTON - Colombia's largest guerrilla group is losing strength, the U.S. ambassador to Colombia said Tuesday, and may weaken further as U.S. involvement in the South American conflict deepens.

  U.S. Special Forces will ''be on the ground in a few weeks'' to help train Colombian counterinsurgency units, and Washington plans to help Colombia track and target leftist and rightist outlaw commanders, U.S. Ambassador Anne W. Patterson told an academic conference.

  Patterson said the Bush administration will not ''stray too far'' from its principal goal of combating the narcotics trade in Colombia, nor will it seek to lift a cap of 400 U.S. soldiers or 400 U.S. civilian contractors in Colombia at any one time.

  But Patterson said greater U.S. military involvement and intelligence sharing with the Colombian government will help the nation combat the main leftist insurgency, the Revolutionary Armed Forces of Colombia, or FARC, and two other outlaw armies.

  ''The FARC is getting weaker militarily,'' Patterson said, explaining that the insurgency can no longer amass the number of armed units for assaults that it once could.

  She said that helping Colombia fight its outlaw armies might in the long run help it achieve better conditions for eventual peace talks.

  ''If these terrorist groups are under military pressure, and some of their more intractable leaders are removed, and some of the [armed] children and teenagers can be persuaded to leave their ranks, a negotiated settlement may once again become possible,'' Patterson said at a conference sponsored by the Center for Strategic and International Studies, a Washington think tank.

  Peace talks with the FARC broke down in February, forcing then-President Andrés Pastrana to end a three-year-old demilitarized zone that had become a haven for the guerrillas.

  Growing frustration over banditry and rampant kidnapping prompted Colombians in May to elect Alvaro Uribe, a civilian hard-liner, who assumed the presidency Aug. 7.

  Since his election, Uribe has declared regional ''states of commotion'' that curb rights, set up a network of civilian informants and outlined a policy of ''democratic
  security'' designed to extend the reach of the central government to lawless outlying areas.

  ''The state's lack of capacity to provide security for its people has left power vacuums over its territory that are filled by the terror of insurgent groups or of the no less brutal so-called paramilitary groups,'' Colombian Defense Minister Marta Lucía Ramírez told the conference.

  Uribe's election coincided with a vote on Capitol Hill that broadens U.S. involvement in Colombia beyond a drug war. It allows unspent U.S. assistance to be used for combating insurgents as well.

  While the United States has dramatically increased its assistance to Colombia, the third-largest foreign recipient of U.S. aid after Israel and Egypt, the South American nation is the size of France and the United Kingdom combined and ''U.S. helicopters and intelligence will not in themselves enable Colombians to eliminate terrorism,'' Patterson said.

  She said U.S. trainers would help the Colombian army protect a key oil pipeline, which was bombed so repeatedly last year that it cost the nation $500 million in lost revenue.

  ''Colombia has enormous potential in petroleum production but can realize it only if the security situation can be stabilized enough so that exploration can take place,'' she said.

  The nation is now supplier of about three percent of current U.S. oil imports.