Lax security at Energy Department nuclear weapons laboratories has allowed agents from Iran, the Soviet bloc and other countries to gain access to sensitive research facilities, congressional investigators reported Tuesday.

The visits took place at the Los Alamos and Sandia national laboratories in New Mexico and at the Livermore laboratory in California, where scientists conduct advanced research on nuclear weapons and the Strategic Defense Initiative, known as "Star Wars," according to the report by the General Accounting Office.The foreign visitors included specialists from the Soviet bloc, China and nations believed either to have nuclear weapons or are seeking to develop them, including India, Pakistan, Israel, Brazil and Argentina.

Also allowed into the plants were visitors from both Iran and Iraq, the report said.

Officials at the FBI and CIA declined to discuss possible espionage investigations stemming from the incidents, and it was not clear whether any secrets had been lost, according to the report by the agency, the investigative arm of Congress.

Formally delivering the report to the Senate Governmental Operations Committee, the GAO's senior associate directgor, Keith Fultz, said that his inspectors "could not determine if sensitive or classified information has been lost" but added that "because of . . . weaknesses in DOE's foreign visitor controls, we have little confidence that adeaquate protection of weapons-related information and technology is achieved."

One member of the committee, Sen. John Glenn, D-Ohio, said that "there may be a strong possibility that we will today only be looking at the tip of a monstrous iceberg."

The Energy Department "generally does not follow its own requirements and obtain background information on foreign visitors and assignees from communist and other sensitive countries," the GAO report said.

As a result, it said, "suspected foreign agents and individuals from facilities suspected of conducting nuclear weapons activities have obtained access to the laboratories without prior DOE knowledge," the report said.

It found the department "allows foreign nationals from communist and proliferation risk countries into the laboratories to discuss subjects that could assist nuclear weapons programs."

"I can tell you from my point of view, I personally know of no prolonged visit from any foreign national," said Nigel Hey, spokesman for Sandia.

Hey said foreign visitors must make formal application through the Department of Energy . . . "so actually you can see that the actual procedure that's followed to clear access to Sandia is in the hands of the Department of Energy."

He said further comment would be impossible without first seeing the GAO report.

Congressional investigators found that of the 6,700 foreigners allowed to visit the weapons laboratories between January 1986 and September 1987, 222 were from communist countries and 675 were from other countries deemed to be espionage or proliferation risks, including 25 from Iran and two from Iraq.