Japan has decided not to send mine sweepers or coast guard vessels to help protect sea lanes in the war-torn Persian Gulf, but will finance a new "precision navigation" system to assist ships of all nations in avoiding mines there, Japanese and U.S. officials said yesterday.

The Japanese program, announced by the government of Prime Minister Yasuhiro Nakasone this morning in Tokyo, also will include an increase in Japan's share of the cost of keeping 50,000 U.S. military personnel in Japan as a way to compensate the United States for the cost of its naval escort and mine-sweeping operations in the gulf. {The amount of the additional aid to the United States had not been determined this morning, Washington Post correspondent Fred Hiatt reported from Tokyo.}

The Reagan administration, which has urged Japan to share the burden of protecting the oil lifeline in the gulf, is accepting the plan as a "first step" in this direction. Nakasone had hoped to do more, U.S. officials said, but since his five years as prime minister come to a close later this month, "this was the best he could do."

More than half the petroleum that fuels Japan's industry comes from the gulf. With U.S. and West European naval and air forces protecting gulf oil shipping -- with much of it bound for Tokyo's factories -- the growing international call for Japanese contributions has embarrassed the Nakasone government.

Nakasone promised President Reagan in their Sept. 21 meeting in New York that he would make a decision about gulf assistance before leaving office this month. The measures being announced today, following Nakasone's decision last week to purchase a U.S. jet for the next generation Japanese fighter-bomber rather than build a Japanese plane, were described by officials on both sides as Nakasone's final gesture to Reagan and to U.S.-Japan relations.

A senior U.S. official said the United States had never spelled out what kind or scope of gulf aid it was seeking from Japan, largely because it understood that sending any of the 31 Japanese mine sweepers or even coast guard search-and-rescue vessels would be a ticklish matter under Japan's 1947 constitution.

In that document, drawn up by Gen. Douglas MacArthur, Japan renounced war and use of force for all time.

Objections from elements of the ruling Liberal Democratic Party as well as anticipated trouble from opposition parties and the Japanese public stymied efforts to dispatch any ships now, according to Japanese sources.

U.S. officials held out hope, however, that a new prime minister might be able to move in this direction after more extensive preparation of politicians and public than Nakasone was able to accomplish.

The Japanese-financed navigation system is an extensive network of shore electronic receiving stations in the Persian Gulf and transmitters aboard ships.

It would permit ships to pinpoint their locations with greater precision than at present, thereby staying away from known danger areas and within sea lanes swept by mine sweepers.

The system, made by the U.S.-British combine, Racal-Decca, is expected to cost more than $10 million.

Other Japanese measures announced today are increases in economic aid to Oman, a strategic country at the mouth of the gulf, and to Jordan, a key country in Mideast peace efforts. Oman will receive $200 million and Jordan $300 million. Another measure is a willingness to pay for U.N. peace efforts in the gulf.

The increased financial support for U.S. military in Japan, is seen as Tokyo's way of indirectly subsidizing U.S. efforts in the Persian Gulf.

Japan now pays about $1.5 billion toward maintaining U.S. forces in that country; the U.S. pays about $3.2 billion.

To pay more, according to the Japanese government, the 1960 "status of forces" agreement must be revised or some other new framework for this sort of assistance established.