Five years and $3 billion into the most aggressive counternarcotics operation ever here, American and Colombian officials say they have eradicated a record-breaking million acres of coca plants, yet cocaine remains as available as ever on American streets, perhaps more so.

''It's very disturbing,'' said a senior State Department official traveling here with Secretary of State Condoleezza Rice, who is on a five-day tour of the region.

Colombian traffickers still provide 90 percent of the cocaine used in the United States and 50 percent of the heroin, just as they did five years ago, the government says. ''Key indicators of domestic cocaine availability show stable or slightly increased availability in drug markets throughout the country,'' the White House drug policy office acknowledged in February. Officials added that prices have remained stable and purity has improved.

Several senior officials said they were perplexed and disappointed. The White House report said, ''There is little interagency consensus for this disparity.''

Over the past five and a half years, the United States has spent nearly $3 billion on programs to fight drug trafficking, train the Colombian military to battle insurgents who control much of the drug trade, and improve institutions of government. The initiative is called Plan Colombia.

The centerpiece of this effort has been the use of a small air force, 82 aircraft, to spray herbicide on coca plants grown on small plots and large plantations across the country. Over five years, more than 1.3 million acres of coca plants and 52,000 acres of opium poppy have been destroyed at great cost. Traffickers have shot down at least five of the planes; three lives have been lost.

Theories abound on the reason for the disparity between the eradication numbers and the availability estimates, and on how to deal with it. Luis Alberto Moreno, the Colombian ambassador to the United States, said he believed the Colombian enforcement teams should be uprooting the coca plants instead of spraying them with herbicide.

The senior State Department official said he suspected traffickers were hoarding vast supplies of cocaine and doling it out slowly. Representative Dan Burton, the Indiana Republican who is chairman of the Western Hemisphere subcommittee, said he thought the Colombians should be using a more powerful herbicide. And the White House drug policy office hypothesizes that the government's data on drug cultivation may be inaccurate.

Gen. Jorge Daniel Castro, director of the Colombian national police and one of Colombia's primary experts on the issue, described the Plan Colombia drug-enforcement paradox as ''a complex phenomenon'' but added when pressed that he believed that the traffickers were simply replanting the coca and opium plants almost as soon as the spray planes left.

Even with the contradictory results from the first five years, the Bush administration is asking Congress to extend Plan Colombia for at least one more year. The president's budget proposal asks for another $734 million next year on top of the $2.9 billion already spent.

A senior State Department official who is involved in the Colombia program said, ''Give us another year or so and see if there is any effect.''

At a news conference here on Tuesday, Ms. Rice said Washington had no intention of reassessing the program, adding that such a move would most likely take a long time to see results in the United States.

Mr. Moreno said the traffickers had ''improved their productivity, but I think we are getting close to the tipping point.'' He is lobbying Congress to renew the financing.

A Congressional delegation was in Colombia this week to ask whether Plan Colombia was producing results commensurate with its cost. ''We want to make sure the money we are spending is well spent,'' said Mr. Burton, whose subcommittee will consider the State Department budget request.

Gregory W. Meeks, the New York Democrat, is another subcommittee member who was on the trip this week, and he too said he wanted to evaluate ''how our money is spent'' -- particularly because ''I know in the neighborhoods where I live, we are just not seeing any effect at all.''

Plan Colombia was born at the end of the Clinton administration, after a decade during which drug trafficking from Colombia grew exponentially. A State Department report in 2000 said, ''The situation has long been challenging, but it has reached crisis proportions in the last few years.''

As the Plan Colombia bill reached his desk in 2000, President Bill Clinton said, ''With this funding, we will be able to support the courageous anti-drug efforts of Colombia, which can, in turn, help curb the flow of drugs in our nation.''

That first year, Colombians, with American help, eradicated 116,000 acres of an estimated 336,000 acres of coca under cultivation -- one-third of the crop. Each mission is a complex operation involving a former military plane converted to crop duster and five or six other aircraft, including helicopter gunships that provide protection. The planes spray glyphosate on the crops; that is the generic name for the herbicide sold commercially as Roundup. It kills the plants but is said to leave no residue in the soil.

Earlier this year, the State Department reported that 2004 had been ''another banner year.'' The spray planes had eradicated 336,248 acres of coca plants. But it seemed a Pyrrhic accomplishment. At about same time, the White House drug policy office reported that 281,000 acres of coca plants remained, an area ''statistically unchanged'' from the previous year.

The apparent reason, American and Colombian officials said, is that the coca bush can grow from a seedling to a harvestable plant in only four months. General Castro said a coca farmer could harvest leaves from a bush ''three, maybe four times a year.'' If a farmer replants his bushes after the spray plane leaves, he may miss only one harvest.

What is more, General Castro said, stabbing at a coffee table as if it were a map of Colombia, ''If we eradicate here, they move there to replant and then there and there.''

''They are always replanting.''

Colombia is roughly three times the size of Montana.

Though the acreage under cultivation has remained stable for the last two years, the White House drug office said production of cocaine dropped by 7 percent last year because many traffickers were harvesting newly planted bushes that could not produce as many leaves.

General Castro was in Washington in part to lobby Congress to renew financing for Plan Colombia, telling members of Congress and government officials, ''We're making progress little by little.''

Mr. Burton, the subcommittee chairman, said he was inclined to favor the president's request to renew Plan Colombia financing.

Still, with a resigned air he observed, ''This is a never ending battle.''

By JOEL BRINKLEY