By CHRISTOPHER MARQUIS and JUAN FORERO
 
WASHINGTON, March 22 — The Bush administration released eradication estimates on Monday showing solid progress in wiping out coca crops in the Andean region, prompting some officials to predict that by the end of this year there will be no significant plantations of mature plants to feed Colombia's cocaine production.

But the price of progress has been high. Some American officials involved in the aerial spraying program in Colombia warn that pilots are increasingly vulnerable to attacks by well-armed guerrillas as they go after plantations in the most remote areas. The downing of four planes and the death of one pilot in Colombia last year have prompted some officials to question the future of the spraying program and the value of security offered by American-trained counternarcotics battalions.

The spraying of Colombia's coca has been hampered by Marxist guerrillas and undermined by intelligence breakdowns between American managers and Colombian forces, according to people who work in the program, internal memos and the report of a crash investigation by the State Department, which manages the program.

The pace of the herbicide spraying slackened late last year as defoliation operations were diverted from regions where guerrillas posed a mounting danger to planes. In all, six spray planes crashed last year, four of them shot down by rebels from the Revolutionary Armed Forces of Colombia, known as FARC. In the last downing, on Sept. 21, the pilot, Mario Alvarado, 33, a Costa Rican, was killed. In 2002, only one spray plane was lost, in an accident.

The estimates on the Colombia fields for 2003, which were prepared by the Central Intelligence Agency, show a 21 percent decline in the cultivation of coca, from which cocaine is derived. In the same period, Peru registered a drop of 15 percent, but Bolivia charted a 17 percent increase. The figures result in a regional reduction of more than 15 percent last year and production totals at their lowest level since estimates began in 1986, officials said.

Colombia is by far the biggest coca producer, accounting for 80 percent of the cocaine reaching the United States.

Administration officials said a milestone had been passed in the drug war.

"If you go country by country, you're seeing significant movement in every measurable area," Robert Charles, the assistant secretary of state for international narcotics, said in a telephone interview. "We're very much at the tipping point right now."

Mr. Charles and other officials acknowledge that the supply of cocaine has been so vast for so long that it is unlikely there will be a discernible effect on American streets for some time to come.

Still, counternarcotics officials in Colombia and Washington say they have virtually eliminated coca production from Putumayo Province and other formerly high-yielding regions.

While the C.I.A. estimates reflect a new methodology this year involving enhanced computer analysis, officials said the progress was real and built on a 14 percent reduction in 2002.

Such projections would have seemed far-fetched when the United States began Plan Colombia four years ago, spending $2.5 billion. The aid later expanded across the Andes.

In Washington, officials released the findings to coincide with a White House visit on Tuesday by the president of Colombia, Álvaro Uribe, whom they praise for taking a tough line against drugs and the armed insurgents who profit from them.

More than 50 pilots, Americans, Colombians and citizens of other Latin American countries, are paid as much as $100,000 by State Department contractors to carry out secretive, military-style missions to defoliate coca and undercut the rebel group, which draws much of its funding from the cocaine trade.

Pilots, former pilots and others close to the program say the crashes could have been avoided had the Colombian Army brigade trained by United States Special Forces to protect planes not been assigned to other antidrug missions. But increasingly last year, they said, the brigade was tied up hitting drug laboratories or otherwise unable to secure coca fields before the crop dusters begin spraying.

"The heart of effectiveness is to use the brigade to secure fields and hitting labs and FARC facilities," said John McLaughlin, who managed the State Department's counterdrug aviation program for 25 years until his retirement in October. "The entire brigade is supposed to be committed. But they used them for other purposes, for one thing or another."

Intelligence glitches have also led managers of the spraying program to send unaccompanied planes into regions crawling with guerrillas. One result is that guerrillas last year struck planes 380 times with small arms fire, twice as much as the year before, officials said.

Gen. Barry McCaffrey, who served as President Clinton's drug czar and is now retired, said the spraying missions appear to be more perilous than military sorties over Iraq. "My guess is it would be more dangerous than flying in Iraq, because on every mission, you're facing people determined to kill you," General McCaffrey said.

In one sense, the pilots are victims of their own success. They are being pushed into more hazardous zones, having graduated from the days of huge industrial-size coca fields, when pilots "would pull the trigger on the spray and keep the trigger depressed until they ran out of chemical," said one official who works on counternarcotics operations.

Coca fields are now often hidden away in rebel-held zones that are easier to defend. Pilots also note that rebels have become more proficient at hitting the low-flying planes. "They would learn planes would come back and they would mark the spot and set up on the other side with RPG's or gas cylinders and just wait," one former pilot said, referring to rocket-propelled grenades or the homemade missiles the rebels favor.

At any one time, according to Mr. McLaughlin, as many as a third of all spray aircraft were out of commission for battle damage repairs. The effect on the defoliation, he said, was that while nearly 30,000 acres were hit each month through August, the monthly total dropped off to 19,000 acres in September after four planes were shot down in a three-month period.

The last downing, on Sept. 21, highlighted the problems.

That morning, Mr. Alvarado, the Costa Rican pilot, was flying in a three-plane formation outside the northeastern town of Tibú when a rebel-made projectile struck his plane. The aircraft spun into the ground at 220 miles an hour, killing Mr. Alvarado.

A State Department report on the crash said that "lack of timely and accurate intelligence directly contributed" to the downing and said the Colombian Army knew of a "major FARC encampment" but that pilots were not informed.

The report said that troops were sent to protect planes, but that there were not enough of them in the zone being sprayed.

One pilot familiar with the episode said both the army and the police knew that the area had a high concentration of guerrillas "but they never told us that."

"We went out there unknowingly," he said. "Afterward, someone from the police, when they were investigating what went wrong, they said, yeah, we knew we had a lot of guerrillas out there."

Christopher Marquis reported from Washington for this article and Juan Forero from New York.