By JUAN FORERO
 
BOGOTÁ, Colombia, Oct. 10 - The number of American military personnel here will double, to 800, in the coming months, based on a weekend vote in the United States Congress.

The action was welcomed by President Álvaro Uribe's government for its fight against Marxist rebels but condemned by human rights monitors, who warned of a sharp escalation in Colombia's conflict.

The 2005 United States Defense Department authorization act, approved Saturday by Congress, also permits the Bush administration to increase the number of American citizens working for private contractors in Colombia to 600 from 400.

The soldiers and many of the contractors will, among other things, develop and analyze intelligence on rebel movements, do surveillance and train Colombian troops in counterguerrilla operations.

American officials who lobbied Capitol Hill to lift restrictions said more American personnel were urgently needed to help Colombia in its nine-month offensive in the south that pits 18,000 Colombian soldiers against the country's most formidable rebel group, the Revolutionary Armed Forces of Colombia. "That requires logistical capabilities, maintaining supply lines, getting food and fuel to the front, providing medical evacuation capabilities," said Adam Isacson, a senior analyst at the Center for International Policy, a Washington group that tracks Colombia. "They need a lot more American personnel to fill those gaps."

Though the United States has contributed $3.3 billion to Colombia, most of it in military aid, Mr. Uribe has lobbied hard for a larger American role in the 40-year-old, drug-fueled conflict.

Lifting the Congressionally mandated limits on troops and contractors, a little-noticed measure in the 5,000-page Pentagon authorization bill, is seen by some political analysts and rights advocates as a major step toward even larger American troop commitments. In the months before the passage by the United States in 2000 of Plan Colombia, a $1.3 billion antidrug initiative, members of Congress hotly debated whether involvement in Colombia could lead to a Vietnam-like quagmire.

"The main concern is two years from now: what is going to stop them from coming back for more, until Colombia becomes one of our most serious military commitments," Mr. Isacson said, referring to American military planners.

The work Americans and others do in Colombia's conflict is perilous. Eleven contractors, American and other foreign nationals, working for American companies under Pentagon contracts have been killed since 1998. Three Americans whose plane crashed in a surveillance mission over rebel territory remain in guerrilla hands 17 months after being taken hostage.

Under Mr. Uribe's administration, violence has ebbed in Colombia, the economy has improved and the security forces have made gains eroding rebel forces and destroying vast fields of coca, the crop used to make cocaine. But combat remains common, and political assassinations and kidnappings occur with staggering frequency.

American involvement is being ratcheted up as the United States steadily increases training for police and military forces in Latin America.

In 2003, American soldiers trained 22,831 Latin American troops and police officers, 52 percent more than in 2002, said a report released last week by three Washington-based policy groups, the Center for International Policy, the Washington Office on Latin America and the Latin America Working Group Education Fund. In Colombia, nearly 13,000 troops received American training, up from 6,477 in 2002.

Even before the new policy in Colombia was approved, American officials and military officers had hinted that support for Mr. Uribe's government would be expanded.

"We will stay the course," Gen. James Hill, the commander of American military operations in Latin America, said last week in Bogotá in a farewell address before he retired. He said that the United States would "assist the Colombian people in ways that are necessary to win the war."