Officials in Bogota said Gore also called off a planned September visit to Colombia, Washington's closest ally in Latin America.

Kreider said that Uribe had not been scheduled to attend the Miami event at the time Gore accepted to address the forum.

But he also stressed that the claims that emerged in recent days in Colombia "are deeply troubling".

Uribe, who has repeatedly denied past links with paramilitary leaders, said he hoped Gore "will closely examine the situation in Colombia".

The mounting scandal has also led US legislators to block $55m in military aid to Colombia.

Assassination plan?

In the latest twist, a Colombian state prosecutor said on Friday  that authorities were investigating an alleged plan to assassinate Gustavo Petro, a leftist senator.

The senator said on Tuesday that ranches owned by the president's family were used for meetings of paramilitary leaders in the 1990s.

On Wednesday, Colombia's supreme court announced it was  investigating the president of the house of representatives and two  other politicians for alleged links with the paramilitary United Self-Defence Forces of Colombia (AUC), which the US considers a terrorist organisation.

Earlier this year eight politicians from parties in Uribe's governing coalition were arrested for alleged links with the AUC.

Colombia's paramilitary groups were organised as private armies in the 1980s, ostensibly to protect landholders from leftist guerrillas, but were later accused of numerous massacres of civilians suspected of leftist sympathies, and of involvement in drug trafficking.

The AUC officially demobilised the last of its 30,000 fighters in April in a deal that guaranteed prison terms of no more than eight years to paramilitaries convicted of crimes.