The illegal self-defence forces, or paramilitaries, have been growing in strength

Rand Corp

"The threat of political and military deterioration in Colombia could soon confront the United States with its most serious security crisis in the hemisphere since the Central American wars of the 1980s," the report says.

It suggests the US should step up assistance to help Colombia strengthen its armed forces and state institutions.

Washington should also work with Colombia's neighbours to contain the risk of violence spilling over and pave the way for a multilateral response force if these efforts fail.

Plan Colombia

The US Congress approved a $1bn programme towards Colombia's counter-narcotics offensive, thorough the purchase of helicopters, equipment and training.

Drug farmers have to be given viable alternatives, says Rand

But the Rand analysis questioned the Plan Colombia strategy which includes destroying drug plantations through aerial fumigation.

It warned that in the absence of an effective crop substitution programme, destroying drug plantations would simply displace growers and increase support for the guerrillas.

"Alternative strategies, such as targeting bottlenecks in the drug refining and transportation network in Colombia, should be explored," Rand said.

It noted that far from having been defeated, the insurgents held the operational and tactical initiative - the largest guerrilla group, the Revolutionary Armed Forces of Colombia (Farc) has been growing stronger since the 1980s.

"The illegal self-defence forces, or paramilitaries, have been growing in strength in areas where the government is unable to provide security to the population," Rand said.

"They will continue to be a factor as long as the conditions that gave rise to them are not changed."

In a reaction, US State Department spokesman Richard Boucher said he had not seen the report but dismissed any plans for a multinational force to combat the problem.