A top US anti-drugs official has accused Venezuelan President Hugo Chavez of being a "major facilitator" of the trade in cocaine.
Mr Walters says Mr Chavez is complicit in the drugs trade
The official, John Walters, said Venezuela had become "a haven" for shipments of cocaine manufactured in neighbouring Colombia.
Venezuela rejects the charges, saying it is the victim of traffickers.
But Mr Walters, speaking on a visit to Colombia, said failure to deal with the problem amounted to complicity.
Mr Walters, director of the US Office of National Drug Control Policy (ONDCP), was meeting Colombian President Alvaro Uribe.
US officials say about one third of Colombia's output of 600 tonnes of cocaine a year now passes through Venezuela, most of it going to America and Europe.
Mr Walters said Mr Chavez had failed to root out corrupt officials or to deny Venezuelan ports and airfields to smugglers.
Such failure, he said, came from more than neglect.
"It goes beyond 'I can't do it' to 'I won't do it'. And 'I won't do it' means that 'I am colluding'," Mr Walters said.
"I think it is about time to face up to the fact that President Chavez is becoming a major facilitator of the transit of cocaine to Europe and other parts of this hemisphere."
Mr Walters' declaration served two purposes, says the BBC's Jeremy McDermott in Colombia.
The first is to support Colombia's President Uribe, a key regional ally.
Colombia's relations with Venezuela have all but disintegrated amid fears that Mr Chavez may start actively supporting Marxist guerrillas who are trying to overthrow the Colombian state.
The second is to berate Mr Chavez publicly for not co-operating with Washington's region-wide drug strategy.
Relations between Venezuela and the US are now so poor that US drug enforcement agents can hardly operate in the country, our correspondent says.