US officials concluded in the 1980s that Pakistan was lying about its nuclear program but muted criticism due to Islamabad's support against the Soviets in Afghanistan, declassified documents showed.

The memos to be released Thursday reveal some of the behind-the-scenes drama between the United States and Pakistan during Ronald Reagan's presidency, foreshadowing current-day debates in the uneasy war partnership.

The documents included an account of a secret mission in June 1982 by a US envoy who confronted Pakistani military ruler Mohammed Zia ul-Haq with a letter from Reagan and said the United States had "incontrovertible" proof that Pakistan was seeking nuclear weapons.

The emissary, veteran US diplomat and translator Vernon Walters, said that Zia was "extraordinarily courteous, relaxed" and explained that he had no knowledge of nuclear weapons development but would check with his subordinates.

"Either he really does not know or is the most superb and patriotic liar I have ever met," Walters wrote to the State Department.

The documents, some obtained after requests under the US Freedom of Information Act, were released to the National Security Archive at George Washington University, which made them available to AFP in advance.

Pakistan tested an atomic bomb in 1998 days after its arch-rival India. The United States banned assistance to Pakistan in 1990 -- soon after the Soviets left Afghanistan -- after concluding that it was developing nuclear weapons.

But Reagan exempted Pakistan from a law requiring sanctions, named after then senator Larry Pressler, even though the memos said that officials knew that the country was moving toward nuclear weapons.

The documents showed that the Reagan administration was genuinely concerned about Pakistan's nuclear program, fearing it would trigger instability, and repeatedly warned Zia that Congress could cut off assistance.

"There is overwhelming evidence that Zia has been breaking his assurances to us. We are absolutely confident that our intelligence is genuine and accurate," then secretary of state George Shultz wrote in a November 1982 memo to Reagan.

But Shultz recalled the "essential role" played by Zia in Afghanistan, where US and Pakistani agents funneled weapons to Islamic guerrillas who successfully fought a Soviet invasion.

"A rupture of our relationship would call into question a central tenet of this administration's foreign policy -- strong support for our friends," Shultz wrote, calling the Afghan effort "the most visible evidence of the US commitment to counter Soviet military thrusts worldwide."

The memos said that as far back as 1982, US intelligence detected that Pakistani agents were seeking suspicious items from countries including Belgium, Finland, Japan, Sweden and Turkey.

Years later, such efforts were discovered to be the work of scientist Abdul Qadeer Khan, the father of Pakistan's nuclear bomb. He is also accused of running a vast international black market of nuclear goods.

One secret assessment said that Pakistan was already believed to have enough for one nuclear weapon by October 1985 with assistance from China.

The US memos acknowledged that Pakistan was unlikely to comply with US pleas on its nuclear program in light of its concern over India, with which Pakistan has fought three full-fledged wars since independence in 1947.

The documents said that the United States was also urging "restraint" from India, which had strained relations with Washington during the 1980s.

Despite the criticism of its nuclear program, the United States resumed assistance to Pakistan to the tune of nearly $20 billion after it again offered support in Afghanistan following the September 11, 2001 attacks.

But relations have repeatedly plunged into crisis due to the presence of Osama bin Laden and other militants on Pakistani soil and Islamabad's fears that US ties will again sour once troops leave Afghanistan in 2014.