April 17 (Bloomberg) -- Colombian opposition Senator Gustavo Petro said paramilitary rebels operated out of farms owned by members of President Alvaro Uribe's family in the 1990s.

A member of the leftist Polo Democratico Alternativo party, Petro said in an address to Congress that Uribe helped create the illegal forces. He said his allegations were based on dozens of documents and testimonies from a former paramilitary member and two government soldiers once active with the militia groups.

Uribe, who governed central Antioquia province from 1995 to 1997, has repeatedly denied involvement with the groups. The discussion in congress of Uribe's past comes amid a broader investigation into alleged ties between lawmakers and paramilitaries that led to the detention of 14 members of Uribe's ruling coalition.

``These were rural organizations that organized to defend themselves, and most of them were unarmed,'' Interior Minister Carlos Holguin said following Petro's address.

The allegations threaten the passage of a free-trade accord with the U.S., said Bertrand Delgado, an economist with IdeaGlobal Inc. in New York. Michigan Democratic Representative Sander Levin, chairman of the House trade subcommittee, said the developments are ``very worrisome'' and Colombia can't count on passage of the agreement, the Washington Post reported Feb. 17.

President George W. Bush said on a March 11 visit to Bogota that his administration is committed to the agreement.

Colombian prosecutors are seeking evidence paramilitary leaders and members of congress had an agreement under which legislators would support the fighters in return for votes.

`Lot of Noise'

The son of a rancher murdered in a 1983 kidnap attempt, Uribe used legal armed militias, known as Convivir, created by the national government to combat terrorism as governor of Antioquia, one of Colombia's most violent provinces.

Petro said the militias were infiltrated by death squads.

Political uproar may factor in the debate among U.S. lawmakers as they consider additional anti-drug and military aid this year under the so-called Plan Colombia program, designed to fight narco-guerrillas.

``This will create a lot of noise, but it will pass,'' said Delgado.

About 30,000 paramilitary fighters accepted Uribe's offer to hand in their weapons in exchange for cash and reduced jail terms, part of his plan to ease violence in Colombia caused by four decades of fighting among paramilitaries, guerrillas and government troops.

To contact the reporter on this story: Helen Murphy in Bogota at hmurphy1@bloomberg.net

Last Updated: April 17, 2007 22:09 EDT