Colombia's cocaine and heroin production is set to rise by as much as 50% as the US-backed drug war flounders, due largely to the growing strength of Marxist rebels, says the report from the General Accounting Office (GAO).

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"The cocaine threat from Colombia has worsened since 1996 and could deteriorate even further within the next two years but also Colombia is now the primary provider of heroin to the eastern United States," it added.

Colombian gangs smuggled 165 tonnes of cocaine into the US in 1998, double the 1995 figure, and output is set to rise to 250 tonnes by 2001.

The projection is a severe setback for the United States which sent a record $280 million counter-narcotics aid to the Colombian police and army this year. Even more aid has been promised for next year.

The rebel threat

Cocaine crop is spiralling

The report - presented to Congress last week but only published on Thursday - blames the increased flow of drugs on the growing power of Colombian guerrilla groups and their involvement in the drug trade.

As part of its efforts to stem drug-trafficking, the GAO said US officials were now routinely sharing intelligence on guerrilla activities with the Colombian military.

"Recent offensives by the insurgents ... suggest Colombian security forces will be unable to conduct effective anti-drug operations in regions where guerrilla forces dominate and control the area," the GAO said.

US officials estimate that two-thirds of units of the Revolutionary Armed Forces of Colombia (FARC), the largest rebel group and one-third of the National Liberation Army are now involved in the drug trade.

The Colombian rebels earn up to $600m every year from taxing, protecting and smuggling drugs, US officials believe. The money is alleged to go to buy more arms and extend rebel power even further than the 40% of the country they already control.

The rebels have consistently denied direct links to drug-trafficking and say the United States is using anti-drugs operations as a pretext for taking an active role in counter-insurgency efforts.

Colombia's President Andres Pastrana is conducting slow-moving peace talks with FARC, but clashes continue.

US State Department officials met FARC officials in Costa Rica last year to discuss crop substitution plans but the GAO report stated categorically that "the United States will not support alternative development programmes in areas until they are under government control."