Colombia's cocaine traffickers are sharply expanding their cultivation and refining of opium as part of an ambitious campaign to break into the world heroin market, according to U.S. antidrug officials.

The increasing diversification of Colombia's drug cartels, which face an apparently static cocaine market here, is a new and potentially worrisome development on the drug front, some U.S. officials said. Over the past few months, Colombian traffickers in the United States have begun wholesaling heroin with purities as high as 90 percent. That is an unusually high-grade product that officials believe is being refined in Colombia from fledgling opium poppies planted in remote mountainous regions of the South American nation.

U.S. officials said they also have received recent intelligence reports that Colombian cartel operatives have established contacts with heroin trafficking groups in the "Goldon Triangle" of southeast Asia, leading to what has been described as an international "transfer of technology" in the narcotics industry. Some of the Colombian-refined heroin seized this year has the precise chemical makeup of high-quality "China white" heroin, prompting Drug Enforcement Administration (DEA) officials to conclude that the Colombians have imported Asian chemists to assist in the production.

Indications of the trend emerged last month with the conviction of reputed Medellin cartel operative Humberto Sanchez on heroin distribution charges in New York City and the Nov. 29 seizure of nearly 2 pounds of high-purity Colombian heroin at the apartment of the San Juan Municipal Ballet director in Puerto Rico.

In the Puerto Rican case, DEA agents charged that the ballet director and several other local figures, including a Puerto Rican treasury agent and the son of a Puerto Rican legislator, were working for a Colombian drug ring that was smuggling large quantities of heroin and cocaine onto the island, with some of the drugs destined for New York.

"It's a very significant case. . . . It's a clear indication that Colombians are using their cocaine labs to process heroin," said Felix J. Jimenez, the chief of the DEA's heroin desk. "We are gradually seeing Colombian cocaine traffickers getting involved in the manufacture and distribution of heroin in the United States."

Jimenez emphasized that Colombians account for only a small portion of a multibillion-dollar heroin market that in recent years has been dominated by Chinese and other Asian trafficking groups. Nevertheless, the entry by the Colombians is considered especially troubling because their sophisticated distribution networks in the United States could make heroin accessible to a far larger clientele than has been the case in the past, some officials said.

"It is a situation that needs a lot of monitoring," said one U.S. antidrug official. "If they decide to go large-scale, with their infrastructure, it could be very significant."

The development is also likely to fuel a debate among Bush administration officials over whether heroin is poised for a comeback that could make it the new "problem drug" of the 1990s.

While U.S. antidrug officials have focused almost exclusively on cocaine in recent years, world production of opium has soared with bumper crops in Myanmar (formerly Burma), Pakistan and other Asian countries in the past few years. Seizures of heroin entering the United States were about 800 pounds in 1987, topped 1,500 pounds through the first nine months of this year and are expected to reach a new record by the end of the month.

So far, there is no evidence this has translated into an increase in the population of American heroin users, an aging group estimated at between 500,000 to 750,000 addicts, said Herbert D. Kleber, deputy director of national drug control policy.

But some law enforcement officials fear that heroin is becoming so cheap and available that it will eventually start attracting a new clientele, particularly among crack smokers who are unable to sustain the frenetic lifestyles and frequent binges associated with their crack cocaine habits.

The Colombian entry into the field appears to be an effort to exploit such a change in drug habits, driven in part by signs that the cocaine market may now be contracting substantially, antidrug officials said. About 12 to 18 months ago, the Colombians began "experimental" plantings of opium poppies, and earlier this year, Colombian police reported seizing their first heroin.

But little was publicly known about the Colombian involvement until last month with the trial of Sanchez, a 30-year-old chemist who allegedly worked directly for slain Medellin cartel leader Jose Gonzalo Rodriguez Gacha. Federal prosecutors in New York charged Sanchez had supervised a network of more than five Colombians who had arranged to smuggle and distribute heroin that had been refined from opium fields planted by Rodriguez Gacha in Colombia.

"There was testimony that he {Sanchez} viewed heroin as more lucrative than cocaine," said Alexandra Rebay, the assistant U.S. attorney who handled the case. "We had Sanchez on tape discussing it."

Special correspondent Douglas Farah in Bogota contributed to this report.