A major international conference to deal with the growing plight of hundreds of thousands of Indochina refugees is being planned for mid-July, according to well-informed U.S. and U.N. officials.

Detailed plans for such a meeting, probably to be held in Geneva, are to be announced by U.N. Secretary General Kurt Waldheim before the end of the week, U.N. officials said.

Previous conferences on the Indochina refugee crisis, which has been festering for months, have ended with little more than vague promises, and countries such as Malaysia and Thailand have adopted increasingly harsh policies against the refugees.

Officials planning the July meeting have been extremely cautious out of fear that an inconclusive conference could push countries on the front-line of the refugee exodus into even more drastic steps than those already taken.

Malaysia already is turning away newly arriving refugees, and Thailand began pushing Cambodian refugees back into their homeland two weeks ago. The number of Cambodians believed to have been forced back to an uncertain fate is believed to exceed 50,000.

More than 300,000 Indochinese refugees now are in camps, mostly in Thailand, Malaysia, Hong Kong and Indonesia, and about 300,000 others have been resettled in permanent homes. The flood of people leaving Vietnam has been increasing in recent months, however, and officials say that six have been arriving in so-called first-asylum countries for every one resettled.

Malaysian Prime Minister Dato Hussein Onn in a letter to Waldheim earlier this week threatened to take dramatic steps if the 76,000 Vietnamese refugees in U.N. camps in Malaysia are not resettled "within a reasonable time frame."

"Malaysia will not allow itself to be saddled with the residues of these boat people," the prime minister warned. "Therefore, if they are not accepted by resettlement countries or their country of origin, we will have no choice but to send them out, which is the only alternative to their being left to rot in the camps."

British Prime Minister Margaret Thatcher first issued the call for a top-level international meeting, and she has been supported by France and the United States. U.N. officials say, however, that Waldheim has been reluctant to announce the meeting until he is certain that it will produce results in the form of sharp increases in funding for the beleaguered U.N. refugee program and promises of increased numbers to be accepted for resettlement.

Approximately 10,000 refugees a month are currently being taken out of camps in Indochina for resettlement, 7,000 of those to the United States and the rest primarily to France, Australia and Canada.

Most of those leaving Vietnam are either Chinese, and U.S. officials say that as many as one million more may leave. Even if the number being resettled is doubled to 20,000 monthly, which officials concede may be the best that could be hoped for, it would take five years to find permanent homes for such a huge number.

Vietnam announced yesterday that it would be willing to attend a new conference but only if it is held under the auspices of the U.N. High Commissioner for Refugees on subjects outlined by Hanoi.

It is not at all clear that other nations would want to limit the conference ot Hanoi's definition of the Issues, and this is believed to be among the questions delaying formal announcement of the conference.

A signal that any such conference could degenerate into name-calling came yesterday from Vietnam when it coupled its statement of willingness to attend a meeting with a sharp attack on the United States and China as being the causes of upheaval in Indochina. Western and Southeast Asian countries have sharply accused Vietnam of forcing out its citizens who are of Chinese descent.

A U.N. spokesman said yesterday that officially all that can be said is that the "possibility of a mid-July meeting is most likely," but that there was pressure by some countries to have it earlier and some later.

State Department spokeswoman Jill Schuker said yesterday that the United States was dealing with the refugee issue at "the highest level." CAPTION:

Picture 1, KURT WALDHEIM. . . reluctant to announce meeting; Picture 2, Boat that brought refugees to Malaysia will be repaired by local authorities who intend to force the refugees back out to sea.