The Japanese government, in an apparent policy shift, seems to be backing away from earlier assertions that it trusts the United States to carry out U.S.-Japanese accords barring storage of American nuclear weapons in Japanese territory.

The government has asked for clarification from Washington of the latest report -- that a sign warning of nuclear contamination is still present at an ammunition dump -- suggesting that American nuclear weapons may have been brought into Japan in violation of the agreements.

Faced with earlier claims that American nuclear devices have been present in Japan, the government had been saying only that it trusted that the United States has always honored its agreement.

The seeming change in policy follows a week of intense pressure on the government to answer questions about a series of allegations that nuclear weapons have either been stored in Japan or brought into ports aboard the American warships that call here.

The press and opposition members in parliament have persistently demanded thorough investigations, including responses from American officials. But the government so far has refused to respond to the substance of any of them.

The latest report surfaced in the parliament yesterday when an opposition member, Yanosuke Narazaki, disclosed that a sign warning of possible nuclear contamination has been discovered at former U.S. ammunition dump near the city of Kita-Kyushu. He said the sign is still present even though the ammunition dump was returned to Japanese control in 1972.

Narazaki also said he has evidence that U.S. nuclear weapons have been stored at U.S. ammunition dumps near the naval base at Yokosuka and at Kure, near the city of Hiroshima in western Japan.

Narazaki is a member of the United Social Democratic Party, a small group whose views are rarely heeded by the govenment.

A Foreign Ministry spokesman confirmed today that a request for clarification of the nuclear contamination sign has been forwarded to the U.S. Embassy here, but there was no explanation why it is being treated differently from the other cases.

A high-ranking government official today reaffirmed that there will be no similar request to the United States for clarification of a claim from former U.S. officials that the U.S. Navy in 1961 stored atomic bombs on a landing ship anchored off Iwakuni, the site of a U.S. Marine Corps air station.

The ship's presence was disclosed by a former Pentagon official, Daniel Ellsberg, and confirmed by two other former high-ranking U.S. officials.

The chief Cabinet secreary, Kiichi Miyazawa, told reporters today that the government will not press the United States for an explanation of that incident. He said the reports, which were published in The Washington Post, concern matters that took place many years ago and do not merit investigations.

The series of reports about nuclear arms began a week ago with the assertion by former U.S. ambassador Edwin O. Reischauer that American warships carrying nuclear weapons were routinely allowed to dock in Japan and pass through Japanese waters.

They have placed the embattled government of Prime Minister Zenko Suzuki in a delicate position. The admission that any of those reports are true would disclose a long-running government deception and cause the Suzuki government to fall, members of his Liberal Democratic Party said privately this week.

Nuclear weaponry is an extremely sensitive and emotional issue in Japan and governments for two decades have insisted that American nuclear arms are neither stored ashore nor carried into port on ships.

A 1960 Japanese-U.S. agreement clearly promised that nuclear weapons would not be brought ashore. Any evidence proving conclusively that they were would amount to a violation of the agreement.

The presence of ship-borne nuclear weapons is a different matter. Japanese governments have claimed that the 1960 agreement also embraced them and that they could not come into Japanese waters or ports without prior consultation between the two governments.

That interpretation is disputed by Reischauer and at least three former Japanese officials knowledgeable about the 1960 negotiations. They have said the United States never agreed that nuclear weapons could not be carried here on ships.

In the light of their statements, ship-borne nuclear weapons would not vioalte the agreement, but proof that they entered Japanese ports would expose two decades of Japanese governments as having lied to the public.