BOGOTA, Colombia (AP) -- A top U.N. anti-drug official called Tuesday for international monitoring of a U.S.-backed aerial eradication campaign against drug crops and rejected claims that the program is not harming peasant farmers.

The statements by Klaus Nyholm, director of U.N. counternarcotics programs here, came amid growing domestic opposition to crop-spraying using chemicals and as the U.S. Congress considers additional drug-fighting aid for the South American country.

A $1.3 billion U.S. aid program is paying for combat helicopters, troops training and crop dusting planes to wipe out coca and opium plantations. Colombia is the world's main cocaine exporting nation and a growing supplier of heroin to the United States.

Nyholm said the United Nations has collected ample evidence that herbicides are being forcibly sprayed on small farmers food plots.

"We know that despite the government's policy, sometimes small farmers' plots are hit as well, and that legal crops such as bananas and beans are being fumigated by mistake," he told a news conference in Bogota.

Nyholm disputed recent comments by Colombian officials that the eradication effort is surgically targeting only large-scale coca and opium plantations run by drug traffickers.

Nyholm said many of the scientific studies dragged out by both proponents and opponents of the forced eradication program are biased. He urged an international monitoring mechanism be created to evaluate the safety and effectiveness of the chemical being used.

Nyholm did not detail his proposal, however, saying only that the United Nations has asked the World Health Organization to get involved.

Nyholm urged that the government place more emphasis on funding alternative development programs, project offering farmers aid to switch to legal crops such as organic coffee.