The United States today sought to ease Japan's latest political crisis over nuclear arms by insisting that it has honored its commitments under the U.S.-Japan mutual security treaty that covers the deployment of nuclear weapons here.

Ambassador Mike Mansfield told Foreign Minister Sunao Sonoda the United States has honored those commitments and "would continue to do so."

Mansfield also observed that the former U.S. ambassador who triggered the latest controversy, Edwin O. Reischauer, "was speaking as a private individual" when he said that U.S. warships have routinely carried nuclear weapons into Japanese ports for 20 years.

Mansfield's comments, released later by the embassy, did not, however, touch on the central question of whether U.S. ships entering Japanese waters actually carry nuclear weapons. He repeated a government policy of refusing to confirm or deny the existence of such weapons anywhere.

Reischauer's remarks have dominated all political discussion here this week. The possible presence of nuclear weapons is the most sensitive issue in Japan. An admission they had been brought into Japanese ports would probably cause the government of Prime Minister Zenko Suzuki to fall and would call into question the mutual security treaty which provides Japan with the U.S. "nuclear umbrella."

The government has said for 21 years that any introduction of nuclear weapons on U.S. vessels would require advance consultation by the two governments, and it is trying to maintain that posture despite the contridiction of Reischauer, who was ambassador here from 1961 to 1966.

Under mounting criticism, Suzuki again insisted that any port calls by nuclear-armed ships would require prior consultations.

"I don't intend to change the policy," he said.

Suzuki was sharply questioned about the issue during a luncheon appearance at the Japan Press Club. One questioner said that continued denials of Reischauer's statements made the government out to be "a pack of liars."

The remark did not cause the prime minister to flinch or change his position. He said he would not respond to any question about lying.

A memorandum accompanying the 1960 treaty requires prior consultation before any major deployment of equipment by U.S. forces. Reischauer said the United States had always interpreted that to exclude weapons aboard warships docking here or passing through Japanese waters. But for years a succession of Japanese governments has insisted that nuclear weapons on ships would also require government consultation.

Reischauer's statements have been substantially supported by several former Japanese officials, who have said in interviews that naval weapons were not included in the 1960 understanding.

It is widely assumed, even by some members of Suzuki's Liberal Democratic Party, that U.S. ships do bring in nuclear weapons. But these politicians do not wish to bring the issue to a head now and risk a collapse of the government.

The issue has surfaced before and preceding governments have been able to ride out the controversies through repeated denials.

In 1974, retired admiral Gene LaRoque testified in Congress that U.S. warships do not offload their nuclear weapons "when they go into foreign ports such as Japan or other countries."

The U.S. government responded then, as Mansfield did today, by insisting that it had honored its commitments under the mutual security treaty and related arrangements.

Major Japanese newspapers have called this week for a thorough investigation and clarification, and editorials have strongly questioned the credibility of this and previous governments' repeated denials.