PANAMA CITY, Panama, By LARRY ROHTER -- For nearly a century, the U.S. military has used the jungle terrain surrounding the Panama Canal as a testing ground for explosives and chemical weapons. Now, a little more than a year before those ranges must be handed back to Panama, Washington is balking at cleaning up some of the most hazardous of the ordnance its troops are leaving behind.

The deadly debris consists mostly of bombs and other unexploded munitions that for decades were tested at three ranges near the west bank of the canal. But the dangerous materials either tested or stored here also include lethal nerve gases, mustard and phosgene gases dating back to World War II, biological agents and, for a short time earlier this decade, depleted uranium projectiles.

The United States, which engineered Panama's secession from Colombia in 1903 in order to build the canal, is required to give up all its military bases here by Dec. 31, 1999. With that deadline fast approaching, though, Washington and Panama are locked in an apparently intractable dispute over to what extent the United States must clean up its mess.

The issue was supposed to have been settled by the Panama Canal Treaties, which were signed in 1977 and require the United States to pay for any environmental cleanup. But faced with a task more complicated and expensive than anticipated, U.S. military officials now contend that the treaties in fact exempt them from carrying out the comprehensive cleanup Panama demands.

The treaties oblige the United States "to take all measures to insure insofar as may be practicable that every hazard to human life, health and safety is removed" from all areas being returned to Panama. The treaties also require the United States to "gather and provide information concerning environmental hazards caused by its activities in Panama" and to "provide compensation for irremediable environmental damage."

Panamanian officials say that since 1979, when formal sovereignty over the former Canal Zone was transferred from the United States to Panama, at least 21 people have been killed by U.S. ordnance left on the ranges. The Pentagon contests that figure, saying its records indicate only seven people, all of them scrap dealers digging for metal in off-limits areas, have died.

W. Lewis Amselem, the U.S. co-chairman of a joint working group set up to deal with range issues, acknowledged in September that "a small part of the reverted areas will not be available for unrestricted use." That land may amount to perhaps as much as 7,500 acres, or 2 percent of the territory being handed over. But he and other U.S. officials argue that every "practicable" step to restore the ranges is already being taken and say they simply cannot do what Panama wants.

"The issue is not time and it is not money," Col. David Hunt, the chief treaty implementation officer for the U.S. Southern Command said in an interview in Panama. "The issue is the technology available to do the job safely while protecting the environment."

Panama disputes that interpretation, pointing to successful environmental restoration efforts in other parts of the world, including the United States itself. The cleanup operation here may be proving more costly and time-consuming than initially expected, Panamanian officials maintain, but that does not exempt Washington from its treaty obligations.

"We believe that the only criterion that should prevail is the technical feasibility of a cleanup," said Jorge Ritter, who is both minister of foreign affairs and minister of canal affairs in the Panamanian government, in an interview. "There should not be an economic limitation for the United States to clean up so small an area in relation to those it has used on its own territory and elsewhere."

Panama also complains of what it describes as the unwillingness of U.S. officials to provide information on what types of tests were conducted at what sites and on what dates. That information is needed, they say, not only to assure that the cleanup is comprehensive but also to dispel Panamanian suspicions that defoliants like Agent Orange were also secretly tested here in the 1960s before their use in Vietnam.

To Pentagon planners, the extensive U.S. military presence here and Panama's tropical climate made the Canal Zone and its environs an ideal testing ground for all sorts of conventional, chemical and even biological weapons. The United States still operates a Tropical Test Center here and as recently as 1993 conducted experiments with the deadly herbicides tebuthiuron, glyphosate and hexazinone to determine their efficiency in coca eradication in antidrug efforts in South America.

"They have been telling us they will be making information available for as long as I can remember," said Rodrigo Noriega, a Panamanian Foreign Ministry official. "I asked for a precise list of sites early in 1997, but we still have not been given meaningful information regarding chemical and other testing activities."

Panama's concerns about the ordnance problem are shared by Rick Stauber, once an instructor at the Army's bomb disposal school, who initially was contracted by the U.S. military to assess the extent of the damage. But Stauber later became a consultant to the Panamanian government after, he said in a recent interview, the Pentagon pressed him to limit the scope of his investigation and then rejected his findings.

"As I did my investigation and developed my report, I was finding things that were, to say the least, a little disturbing," Stauber said. "The problem was that the recipient of the report was not eager to see what I was uncovering. There were a lot more problems than they wanted to admit to and I opened a can of worms."

U.S. officials deny Stauber's contention that "a lot of information in my report was changed and set in a biased situation" so it would conform to their own pre-established conclusions. But they acknowledge that it has been, as a senior U.S. official put it, "painfully slow and frustrating getting cooperation" from U.S. military authorities, whose interest in Panama seems to be waning as the date of their withdrawal approaches.

The situation is further complicated by the fact that some chemical weapons tests and exercises took place outside the Canal Zone, most notably in four major installations the Panamanian government made available during World War II. Information about those areas, which were returned to Panama after the war, is especially scarce.

Panama has never had a registry of the chemical tests conducted on its territory, Ritter said, "so we need to be clearly informed by the United States about the testing of chemical weapons in Panama."

While the canal treaties do not obligate the United States to clean up areas outside the former Canal Zone, another treaty does. Both the United States and Panama have joined the international Chemical Weapons Convention, which requires them to halt the manufacture and use of all chemical weapons and to destroy their remaining stockpiles. One section of that pact states that when materials found in one country are determined to have come from another, the original user-country is required to clean up the site.

Hunt argued that for environmental reasons Panama would be wise to leave alone the jungle and watershed areas it is pressing the United States to repair, since any tampering "will affect the canal, rainfall and temperatures." He and other U.S. officials also maintained that whatever chemical weapons may have been buried, sunk or lost in Panama no longer pose a danger because they have dissipated.

Stauber disagrees.

"The idea of a limited shelf life is nothing but a red herring and a direct contradiction of the truth," he said. "As long as field munitions are in a container, they are still as good today as 50 or 60 years ago."