U.S. trade negotiators indicated today that the Japanese do not take seriously their warning that a trade crisis is inevitable unless Japan opens its doors to more foreign imports.

They gave that assessment of the Japanese response to two days of negotiations in which the U.S. side insisted on drastic and speedy economic changes by Japan to assist world-wide economic recovery.

In direct, frequently blunt encounters with government leaders, the U.S. representatives have called for prompt actions to increase imports in specific ways that amount to wholesale changes in the Japanese way of doing business.

They have contended that rising protectionist demands in the United States will follow if Japan fails to announce specific plans by the end of this year.

American officials said it is too soon to predict what the ultimate response will be, but one added:

"My impression is that the seriousness of the situation has not been fully grasped and appreciated in Japan. We hope that the need for decisive action is understood.

"In general we are trying to impart a sense of deep concern about a situation which is deteriorating rapidly. Those officials who understand the protectionist problems also understand the necessity of a solution."

The U.S. trade mission is headed by Richard Rivers, an aide of President Carter's special trade representative, Robert Strauss, who is scheduled to come here for crucial negotiations later this year. There are indications, however, that Strauss may cancel the trip if the preliminary talks that began Friday do not evoke specific promises from Japan.

The Fukuda government's response so far has been to listen politely, make no promises, and recite familiar claims that Japan is not to be blamed solely.

Japan's negotiators have said that the biggest U.S. problems in international trade is its huge oil imports, which throw American trade balances into deficit.

They have also said it will be impossible to promise a specific date for wiping out their country's large surplus, as demanded by the American side. And they have not been willing to talk about the radical change in import procedures and tariff reductions that the United States is calling for in unusually pointed language.

The result has been two days of frosty discussions, with the Japanese expressing irritation at the American pressures.

The American official today denies that the steps proposed for Japan are "demands".

"They are in the nature of friendly advice between friendly countries," he said. But he said Japan must pay some dues to preserve a liberal trade system from which it has benefited.

"Japan is a country which has prospered and flourished under an open trade system," he added. "That system is now in jeopardy and Japan could take some steps to reduce that threat."

He conceded that it is "extraordinary for one country to advise another country on its domestic economy. But these are extraordinary times and require extraordinary measures."

The American officials disclosed that they expect a specific response from Japan before the end of this year in time to head off a new round of protectionist fervor when Congress reconvenes in January.

Asked what penalties would be inflicted on Japan if nothing happens, one official said, "There will be no penalties. There is nothing punitive about this.We come as a friendly ally . . . suggesting that protectionism is rapidly becoming unmanageable . . . and that it could react against Japan's own interests. Our interest here is to avert that result."

He revealed the full scope of U.S. suggestions for increasing Japan's imports. Among them is a request to eliminate 27 import restrictions that he said are "illegal" under world trade rules and a thorough overhauling of the import system to reduce non-tariffbarriers.