Depleted uranium is a by-product of nuclear fuel production. It can be used to strengthen ammunition and enable it to penetrate armour.

The disclosure comes at a sensitive time for South Korea, which said in September some of its scientists had enriched a small amount of uranium in 2000 and separated plutonium in 1982.

The government said those tests were conducted by scientists purely out of curiosity, although the IAEA said the failure to disclose them was a matter of serious concern.

South Korea is involved in international efforts to get communist North Korea to give up its nuclear weapons ambitions but the North has said it would not resume talks until an investigation of the South's tests was complete.

South Korea made anti-tank munitions with material derived from the conversion of depleted uranium in the mid-1980s, Jo Seoung-soo, a lawmaker from the opposition Democratic Labour Party, and the Green Korea United group told a news conference

Doing so without disclosure broke an agreement with the U.N. nuclear watchdog, they said.

"The use of the material in anti-tank munition requires conversion of depleted uranium and not reporting it is in violation of the safeguards," said Seok Kwang-hoon, spokesman for the Green Korea United environmental group.

Jo and Seok said the munitions-making at government laboratories between 1983 and 1987 was not aimed at producing nuclear weapons.

"But this is a violation of the IAEA safeguard agreement, and the government's failure to disclose it hurts South Korea's credibility," Jo told reporters.

The government official said the IAEA was notified in 1987 when the programme was scrapped. "No reporting before that had been required," he said.

Another government official said the development of the munitions had "very little to do with the IAEA".

The use of depleted uranium in munitions did not involve conversion of uranium, but a simple reshaping of the material and that process carried no reporting requirement, the second official said.

"This has absolutely nothing to do with nuclear weapons," he said.

The IAEA will report in November on its findings on South Korea's admission to enriching uranium and separating plutonium after inspections in South Korea.