Japan has generally consented to a U.S.-sponsored plan to tighten the financial squeeze on Iran according to informed sources here.
The sources said the United States is seeking support for a package of measures to induce Japanese and European private banks to stiffen their financial dealings with the Iranian government.
The U.S. campaign is separate from the effort to impose broad economic sanctions against Iran, but has the same purpose of exerting economic pressure to force a release of the hostages in Tehran.
The details of the package have not deen disclosed and some parts are still being negotiated, both here and in Europe, the sources said.
But it was understood that one measure would be pressure on the private banks to insist that Iran make up-to-the minute payments on its millions of dollars of loans from those banks, instead of letting the payment deadlines slip by several days.
Such a move could make new financial trouble for Iran, the sources said, because of that country's difficulty in paying its large foreign debts. At present, the sources added the United States is not calling for declarations of default on those loans by the large international banks, but that possibility is not ruled out.
This and other measures have been promoted by the United States since mid-December as a campaign to support the U.S. freeze on Iranian assets. The United States has privately complained that foreign banks in allied countries have in effect helped Iran to circumvent the financial squeeze, but has not spelled out in public what it wanted the banks to do. The Japanese government was initially reluctant to oblige the United States, in part because it fears retaliation by Iran, which supplies 12 percent of this country's oil, and in part because it doubted some of the allegations made privately by U.S. negotiators.
But Japan gave its general consent, according to government officials, during a visit here recently by Deputy Treasury Secretary David Carswell, who had detailed discussions with Finance and Foreign Ministry officials.
Carswell said he was encouraged that "Japan would keep step with European countries in its financial and economic relations with Iran."
That vague statement, government officials said, means that Japanese government will match any actions to curb banks dealing with Iran that are imposed by European governments.
U.S. officials said that in subsequent discussions the Japanese government had been "very cooperative" in supporting the U.S. position.
There were some indications that the United States has softened its position on the financial dealings since they were first explained privately in Paris by Secretary of State Cyrus Vance in mid-December.
Japenese sources said that the United States initially had proposed that several Japanese banks declare defaults in the case of some Iranian loans that were not paid up. That is not now part of the U.S. package, the sources said.
A Japanese newspaper also had reported that Japan had been pressed to freeze Iranian assets in this country, to stop granting new loans to Iran, and to prevent Iran from shifting dollar deposits into other currencies. It could not be confirmed whether those are or ever were part of the U.S. proposal.
Meanwhile, it was learned that Japan also have been prepared to go along with the expected U.S. request at the United Nations for broad economic sanctions against Iran, despite its fear that Iran might retaliate by shutting off all oil shipments to this country.
A government official said that Japan's participation in sanctions would be "a big problem" politically in this country because of the widespread public concern that such a move would endanger oil supplies.
He said it would also evoke criticism from opposition parties in the parliament next month, with critics arguing that the government had gone too far in supporting the United States at the risk of losing supplies of vital oil. Japanese companies have been negotiating with the National Iranian Oil Co. for a 30 percent increase in oil from that country in 1980 but negotiations are now suspended.
The government officials said that, despite the domestic opposition, Japan would have gone along with the U.N.-imposed sanctions because support for the United Nations is a cornerstone of Japanese foreign policy and because the U.N. is a popular institution in this country.
Japan's biggest worry, he added, is that the movement for U.N.-imposed sanctions might fail and the United States would then call upon its allies to impose sanctions unilaterally. Such a move would be widely unpopular in Japan, he added.
The principal casualty of sanctions, from the Japanese viewpoint, would be a giant petro-chemical plant that a Japanese company has been constructing in Iran.