Japanese Premier Masayoshi Ohira yesterday rebuffed U.S. criticism of heavy Japanese purchases of Iranian oil as Secretary of State Cyrus Vance met with a cautious European response in his efforts for concerted action to free American hostages in Tehran.

Ohira, answering U.S. charges that Japan had acted with "unseemly haste" in buying most of the high-priced oil that Iran is no longer shipping to the United States, said Tokyo "must make every effort to secure the oil imports it needs."

The premier said in Tokyo that Japan might consider restricting oil imports from Iran but only "If the situation worsens."

The U.S. -Japanese exchange comes as Washington has embarked on a new diplomatic offensive to seek a unified stance among its closet allies to put pressure on Iran, possibly including steps to isolate Tehran economically.

Secretary of defense harold brown carried the U.S. campaign to the NATO meeting in Brussels yesterday, where he bluntly told members of the Atlantic Alliance that the time had come for "concrete economic and diplomatic steps" against Tehran.

Officials traveling with Vance after meetings with Italian and West German leaders coupled expressions of optimism with clear signals that Washington is not receiving a go-ahead for sweeping sanctions against the Iranians.

The officials said Vance had received encouraging signs of European willingness to cooperate. But they said no commitments had been given and many allies had argued that steps under consideration might not achieve the desired goal of gaining the hostages' release.

Dipolmats in Washington familiar with the new U.S. diplomatic initiative were equally cautious, arguing that Vance was unlikely to win unanimous support for sanctions against Iran. Similar sentiment was also expressed in the French press following Vance's visit to Paris Monday.

Adding to administration misgivings about the way Japan and, to a lesser extent, West European countries are reacting to the Iranian situation are the actions of the international banks that deal with Tehran.

U.S. officials on tour with Vance Monday cited evidence that Japanese are giving Iran technical advice and facilities that will help it pay for essential imports formerly received from the United States.

"If I know Japan," said one American banking analyst, "the banks are operating under some orchestration from the Bank of Japan and the Finance Ministry." The analyst suggested that the banks are facilitating Iranian trade from their branches outside the United States.

"The Japanese feel vulnerable, uncovered and are not fully supporting the United States," the analyst said. "They are nearly paranoid about oil. . . . The Europeans are somewhat in between the aggressive posture of the Japanese.

"The bottom line, according to the Europeans, is that it is not their problem. They get somewhat indignant at the American banks that have tried to pressure them, but they are more understanding than the Japanese."

Apparently such fears and attitudes on the part of allied countries helped to inspire Vance's European tour. One informed source in Washington emphasized that Vance basically is asking, "How far are you prepared to go What can you do technically" and not seeking an immediate answer.

The administration's dealings with Japan exemplify both its frustrations and the cautions give-and-take it is employing in search of concerted international action. Although statements of moral support have been relatively easy to come by, concrete economic action against Iran has proved much more difficult to secure.

State and Energy Department officials said the United States has not tried to promote a complete boycott of Iranian oil and had expected several allies to pick up Iran's surpluses resulting from the U.S. cutoff.

"But when the Japanese companies swooped in and picked almost all of it up," said one State Department official, "that caused some dismay to say the least. They swooped in and scooped it up at outrageous prices."

Japanese oil companies reportedly have bought more than 20 million of an estimated 30 million barrels of oil that previously had been destined for the United States. The cost to the Japanese, according to informed sources, was more than $40 a barrel.

Administration officials also complained that large purchases of Iranian oil on the spot market at prices more than 30 percent higher than the United States had been paying would "give erroneous signals to the market." This could affect the price increases currently being contemplated by the Organization of Petroleum Exporting Countries they argued. OPEC meets next week in Caracas to consider price increases.

These officials pointed out that Japan's government has criticized Japanese oil companies for paying such high prices and asked them to discontinue the practice. The Tokyo government, however, apparently has taken no action against the oil companies.

Both Japanese and U.S. sources said that the Japanese, who are totally dependent on imported oil, were effectively coerced into buying the oil on the spot market as an "entry fee" to negotiations with Iran for longer-term supply contracts next year. About 10 percent of Japan's total oil supply currently comes from Iran.

The current crisis came as Japan's apprehensions about its oil supplies had already been aroused by major cutbacks by the multination companies that deliver almost 65 percent of its oiil, a Japanese embassy spokesman said. t

Taking all this into consideration, administration officials in Washington said Monday's criticism of Japan was "something that had to be said."

"But," added one administration official, "it's been said once and I think that's the end of it. I donT think there's any desire to keep anyone's feet to the fire."