Saudi Arabia, further easing its reluctance about close military ties with the United States, quietly has agreed to provide expanded landing rights and refueling support for U.S. aircraft engaged in Persian Gulf military operations, U.S. officials and diplomatic sources said yesterday.

Some sources also said Kuwait, formerly the strongest opponent among Arab states of superpower military involvement in the gulf, has offered refueling aid for U.S. planes involved in the Navy escort of "reflagged" Kuwaiti tankers.

However, other sources said that, while the United States has held recent talks with Kuwait, they are not sure if understandings have been reached.

As described by the sources, the understanding with Saudi Arabia would go beyond the arrangement under which four U.S. advanced radar-surveillance planes and three aerial tankers have operated from Riyadh since 1980.

The sources said the Saudis, previously unwilling to extend landing rights to U.S. combat planes, will allow carrier-based jet fighters and antisubmarine planes to land for refueling and other logistical aid under what the sources called "emergency" or "in case of need" conditions.

According to the sources, the circumstances under which the ban on combat aircraft will be relaxed deliberately are being kept vague and are likely to be decided case by case.

The situation is "more accurately described as an 'understanding' rather than a concrete agreement in the sense of a document with signatures on it," one U.S. official said. "If one gets too explicit about these arrangements, they are likely to disappear."

That was a reference to domestic and foreign concerns that for years caused the six Arab kingdoms and sheikdoms of the Saudi-led Gulf Cooperation Council to spurn U.S. offers of closer military ties.

Motivating these nations were fears of provoking retaliation from Iran and a desire not to stir up internal nationalist and Moslem fundamentalist forces opposed to overt western influences.

"For that reason, whatever cooperation that has taken place has been conditioned on secrecy," a senior military official said. "On at least one occasion, an offer of assistance has been withdrawn after it was reported publicly." The official declined to elaborate on the offer, but other sources said the reference apparently was to a Washington Post report in June that Saudi Arabia had agreed to use its Airborne Warning and Control System (AWACS) radar planes to watch U.S.-protected tankers in the southern gulf.

The sources said that, after The Post report, the Saudis sought to pull out of the agreement and were coaxed back only after lengthy discussion.

However, the sources said, during the last two to three weeks, Saudi Arabia and the other gulf states have been showing less wariness about military cooperation. One senior U.S. official described the trend as "more of a gradually rising slope rather than a sudden jump."

Others added that, while the gulf states still insist on maximum secrecy, the administration has been so encouraged by their growing cooperation that it has started thinking in terms of what one called "the possibility of establishing an onshore logistical chain" to back up U.S. air and naval forces throughout the gulf.

The sources said the change in attitude appears to have been prompted in large part by such recent incidents as rioting by Iranian pilgrims at Mecca inside Saudi Arabia and the increased number of mines, apparently laid by Iran, being found in gulf waters. The sources said these incidents have made the gulf states increasingly aware that their efforts to mollify Iran through appeasement and keeping the United States at arms length are not succeeding.

In addition, some U.S. officials said, when gulf tension began increasing earlier this year because of the Iran-Iraq war, many gulf states expressed doubt that the United States would be willing to commit sizable forces to protect shipping in the region.

However, these officials said, the way in which the United States has carried out its commitment to protect convoys of "reflagged" tankers has persuaded gulf leaders that Washington is determined not to be intimidated by what one official called "Iran's war of nerves."

Some sources, while acknowledging that cooperation from Saudi Arabia and other gulf states has increased, questioned whether the administration might be exaggerating its importance to win backing for administration plans revealed this week to sell $1 billion worth of arms to the Saudis.

"If you put what we really need -- basing rights for our planes, port access for our ships and joint military planning -- on a scale of 1 to 10, the increase of Saudi cooperation should rate no more than a 3," said an opponent of such sales. Staff writer Molly Moore contributed to this report.