When Pakistan's army chief of staff, Gen. Asif Nawaz, visited Washington in January, he was told by Bush administration officials that despite a ban on U.S. government aid and military sales to his country, Pakistan was free to buy spare parts for its F-16 fighters from commercial sources.

In the English-language newspaper the Dawn, published in Karachi, these assurances were front-page news. "No Ban on Purchase of Spares from U.S.," said a Jan. 18 headline reporting on the Nawaz visit.

This was also news to some members of Congress, who thought that a 1984 law that was triggered in 1990 had completely cut off the flow of arms to Pakistan because of a U.S. judgment that it possessed a nuclear device. In fact, the law had not totally stopped the flow of arms, and the story of what happened offers an illustration of how competing forces in Washington never quit trying to outflank each other, even after a law is passed.

In this case, the legislation had a loophole, and the administration and Pakistan quietly used the loophole to do what some members of Congress thought was prohibited, but which the law had not explicitly prohibited.

At stake are major policy issues, such as how far the United States should go in penalizing a major ally that has decided to build nuclear weapons and how to resolve the constant to-and-fro over foreign policy between Congress and the executive branch. The argument over Pakistan is somewhat reminiscent of another collision between Congress and the White House in the 1980s over the Boland amendment, in which Congress sought to stop U.S. aid to the Nicaraguan contras; the Reagan administration skirted it with private aid and money diverted from arms sales to Iran.

In the case of Pakistan, it was granted licenses to purchase about $120 million in commercial goods in the year after October 1990 when U.S. aid was cut, according to State Department officials. However, these licenses cover several years, and actual shipments amount to $25 million a year. This compares with $176 million in the year before the aid cutoff. Defense industry sources said Pakistan has used most of its purchases to get spare parts for the F-16s.

Sen. Claiborne Pell (D-R.I.), chairman of the Foreign Relations Committee, has said the panel "was never informed of the commercial sales," and Rep. Stephen J. Solarz (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asian and Pacific affairs, called the sales "inconsistent with the spirit if not the letter of the law."

The State Department inspector general's office also is looking into the matter, and Sen. Larry Pressler (R-S.D.), author of the amendment cutting off aid to Pakistan, has asked for a Senate hearing. Secretary of State James A. Baker III has defended the commercial sales as legal, however.

The commercial sales are significant because the intent of the Pressler amendment was to force Pakistan to make a choice between pursuing a nuclear weapon and having a modernized conventional force, including a fleet of F-16 fighter-bombers. By allowing some commercial sales of spare parts for the planes, critics say, the administration has diluted the punishment that Congress intended to impose for going nuclear.

However, administration officials point out that Congress never explicitly prohibited commercial sales in the Pressler amendment. In addition, they say, Pakistan has suffered considerable difficulty because U.S. aid was cut off. Pakistan has been barred from receiving any of the 71 F-16s it has ordered from General Dynamics, and flying time for its existing fleet of 36 planes has been severely limited.

Pressler was in Pakistan the week before Gen. Nawaz visited Washington, and said he obtained his first solid information about the commercial sales on the visit. A few weeks later, he asked Baker about it at a hearing and Baker confirmed that the State Department had permitted commercial sales.

A former government official familiar with the issue said both the administration and Congress share blame. The administration, he said, "didn't want to ask Congress explicitly" for permission to license commercial sales, because the request would have almost certainly been rejected.

At the same time, he said, "the people in Congress who didn't like it didn't raise a stink because they didn't care that much."

The legislation was drafted in 1984 amid worries about Pakistan's nuclear ambitions. The Pressler amendment was a legislative compromise. Some lawmakers wanted to terminate U.S. aid if Pakistan were found to be developing a nuclear device. But others said Pakistan was vital to the U.S. effort to back the rebels in Afghanistan. Thus, a compromise was approved that would cut off U.S. aid only if Pakistan were found to possess a nuclear device, a compromise designed to give Pakistan some breathing room and avoid an immediate aid cutoff.

The law said, "No assistance shall be furnished to Pakistan, and no military equipment shall be sold or transferred to Pakistan" under any U.S. law unless the president certifies to Congress that "Pakistan does not possess a nuclear explosive device" and that U.S. aid will reduce the risk of it possessing one.

However, the law was silent about commercial sales. According to Pell, the "broad language" of the amendment "was specifically designed to cover commercial sales."

But the State Department, in an unsigned legal memo recently submitted to Pressler, maintains that private sales to Pakistan are not "assistance," nor does the United States "sell" or "transfer" the military equipment. Thus, according to the administration, the Pressler amendment "does not apply to government licensing of such arms exports."

"There was a failure on the part of the drafters of the Pressler amendment," Solarz acknowledged, "to cross all the 't's and dot all the 'i's. That gave the administration a loophole, in a way, to allow commercial sales without violating the law. But clearly it violates the spirit."

The amendment was not triggered until 1990 when Bush informed Congress that Pakistan had crossed the line and possessed a nuclear device. At the time, a heated political campaign was underway in Pakistan, and senior U.S. officials went to Capitol Hill seeking a delay in the aid cutoff.

One participant recalled that the administration asked for the delay saying it would be better to deal with a new government after the election. Another participant said the administration did not want to inject the issue into the volatile campaign. However, the Democrats flatly refused and insisted the Pressler amendment be implemented Oct. 1.

As expected, the cutoff produced a negative reaction in Pakistan and relations deteriorated.

Sometime after the aid cutoff, officials at the State Department, the White House and the Pentagon discussed among themselves what Pakistan could and could not do. A decision was made to allow commercial sales on a case-by-case basis for items necessary to maintain and operate equipment Pakistan already had.

Former ambassador Robert Oakley, who was in Pakistan at the time, recalls that on the eve of the aid cutoff, U.S. officials debated every aspect of the relationship, such as whether Pakistan could continue to send students to U.S. military academies.

A key issue was whether Pakistan could purchase spare parts for its planes. These exports would require licenses from the State Department. "We all agreed the U.S. government assistance had to be cut off, and if the Pakistanis then could afford to go for commercial sales, so be it," said a White House participant in the discussions.

"The Pakistanis asked for a clear understanding," this official said, and the response from the administration was, "You will have to go cash-and-carry." The intent was to keep Pakistan's American-made F-16s -- a key part of its conventional defense -- in the air, the official said.

The issue of notification of Congress was, like the law itself, ambiguous. For example, the State Department published a newsletter called "Defense Trade News" last January with an item about the Pakistan policy on Page 11 under the headline, "Other New Defense Trade Policies."

The commercial exports were reflected in an annual summary of expected arms purchases by foreign countries that is sent to Congress.

But these hints of what the administration was doing apparently did not reach the members of Congress. Solarz said if he had been told he would have remembered it. "I was under the impression," he added, "there was a total embargo and no spare parts were going there."