BOGOTA, Colombia (AP) -- Authorities suspect a new threat is lurking in the mountains and jungles of Colombia: Not a new rebel cadre, but altered coca plants that are bigger, faster-growing and produce more of the compound that gives cocaine its kick.

U.S. drug agents are trying to confirm the existence of the purported "super" plant in this Andean nation, the world's prime supplier of cocaine.

But a scientist who advises Colombia's anti-narcotics police says he has already spotted it in prime coca-growing regions, with the new plants towering over conventional ones, which typically reach heights of five feet. Others also say they have seen the bigger, more robust plant.

"What we hadn't been able to do is find evidence of the plant, but now we are finding it," said Camilo Uribe, the scientist.

Uribe said he found the new plants, rising seven to 10 feet, in the Sierra Nevada mountains in northern Colombia and in La Macarena, a region of savannah and jungle in central Colombia.

"They were giant bushes, with really big leaves," Uribe said, adding that the leaves produce higher concentrations of alkaloid, the compound that gives cocaine its high.

Giant coca plants have also been spotted in the state of Putumayo, historically a major coca-growing region in southern Colombia, where locals call the new varieties White Bolivian and Black Bolivian.

In a recent forum in Bogota, Eder Sanchez, a peasant leader from Putumayo, said the Black Bolivian variety is more resistant to herbicides than Tingo Maria, which for years was a favorite among coca growers.

If an herbicide-resistant plant has appeared, it could weaken a pillar of Washington's multibillion-dollar counter-narcotics effort in Colombia: the massive aerial fumigation of Colombia's coca plantations that aims to keep cocaine off U.S. streets by attacking its source.

"We are currently looking at allegations of leaves that are more resistant to spray," a U.S. Drug Enforcement Administration official, speaking on customary condition of anonymity, told The Associated Press from Washington.

Peasant farmers who grow coca, which is "taxed" by rebels and their right-wing paramilitary foes, have tried for years to fend off the effects of herbicide by glazing the leaves with sugar water before the spray planes arrive, or by cutting the bush near ground level after spraying, in hopes it will grow back.

While it is not yet clear if drug traffickers have made a breakthrough in creating a herbicide-resistant coca bush, Uribe said he's seen unofficial reports that suggest they are investing to find such plants.

"The drug traffickers are in a war to protect their business," he told the AP.

Experts say scientists theoretically could manipulate coca bushes to be resistant to the herbicide glyphosate, which is used in the fumigation campaign in Colombia and a version of which is sold commercially by the U.S. company Monsanto under the name Roundup.

Brent Sellers, an expert in weed science at the University of Missouri, said a new strain of coca plant could be developed that is resistant to Roundup, based on the fact that Monsanto sells corn, soybeans and canola that are bio-engineered to resist it. Such products, called "Roundup Ready," permit farmers to spray for weeds without harming their food crops.

Sellers also said that even without manipulation, "if you spray any plant species over and over and over again," it can develop resistance to the herbicide.

Washington has provided $3.3 billion in aid to Colombia since 2000, much of it going toward fumigation. The Colombian Counternarcotics Police says aerial spraying has reduced coca crop cultivation in Colombia by 33 percent since 2001.

However, White House drug czar John Walters said in a recent interview with the AP that cocaine prices on U.S. streets remain unchanged despite the campaign, a sign there is no shortage of the drug.

Copyright 2004 The Associated Press.