Relations have soured between the United States and the new government of the Republic of the Marshall Islands, clouding the future of America's multibillion-dollar Kwajalein Missile Test Range and raising the prospect that the long-negotiated compact of free association between the two countries will never be approved, according to Phillip Muller, the Marshallese deputy secretary of foreign affairs.

Muller, along with Marshallese Foreign Minister Anthony DeBrum, met Friday in Washington with Undersecretary of State James Buckley to lay out their government's demands on the compact and the plebiscite within the islands that was designed to get approval for it.

In an interview here yesterday, Muller said that if the United States refuses to allow the Marshallese to hold the plebiscite before Oct. 1, the Pacific islands government will not renew a rental agreement with the Pentagon for use of the Kwajalein facility. The agreement runs out Sept. 30.

Islands within the Kwajalein atoll are home to the American anti-ballistic missile effort, and the atoll's lagoon is the regular landing site for all U.S. intercontinental missile test shots from Vandenberg Air Force Base, Calif., 4,200 miles away.

Test operations on the atoll are already under some pressure from a sit-in by several hundred Kwajalein landowners, who for more than a month have been occupying several islands in the atoll in protest to financial terms of the proposed compact.

That demonstration at one point so irritated Pentagon officials that they ordered U.S. Army officers on Kwajalein to cut off water for the sanitary facilities used by the protesters.

It took an order of a federal court in Washington to force the Pentagon officials to turn the water back on.

If the Marshallese government joins the Kwajalein land owners in opposition to continued American use of the test facility, it could create serious legal and physical problems for the first test shots of the new MX intercontinental missile, now scheduled to begin next January.

Because of the Kwajalein facility, Washington has spent more than five years trying to end its trusteeship over the Marshall Islands, which has existed since the end of World War II, and replace it with a new association that guarantees future uninhibited use of the missile test range.

Since the draft compact was initialed by representatives of the countries last May, problems in getting it approved have continued to appear both in Washington and in Majuro, the Marshall Islands capital.

The issue that has grown more difficult in the past months is the plebiscite by which the Marshallese were to show their support for the new relationship.

The negotiators agreed in a signed memorandum to seek Aug. 17 as the voting date. Washington officials considered that too soon, citing the need for a voter education program and the desire to have United Nations observers on hand.

The exact wording of the ballot also has been in dispute. The Marshallese want the voters to choose between the free association compact with the United States or "full independence."

In Washington, American officials, particularly those at the Pentagon, are balking at the notion, fearing that if that approach won, the future of the Kwajalein missile facility could be put in doubt.

They have pressed to have the vote between free association or continued trusteeship.

Additional meetings between Marshallese and American negotiators earlier this month in Honolulu not only failed to resolve these issues, but, according to Muller and American government sources, they actually made matters worse.

A Marshallese suggestion that the vote be held Sept. 24 was initially turned down, leading to the island government's threat to not take part in any vote after Oct. 1.

American officials are concerned that Marshall Islands President Amata Kabua may have changed his once-favorable view of the compact, even though he participated in its drafting.

They fear his failure to bring the Kwajalein land owners' protest under control may have convinced him that their opposition to the compact may be symptomatic of a wider distaste for the proposal among islanders.

Muller said yesterday that Kabua always had his doubts about the compact but "said it represented America's last offer and so he agreed to take it to the people and let them vote on it."

American officials pointed out last week that the plebiscite was only one step along the way in ending the trusteeship. Congress has to approve the agreement and then the United States must take it to the Trusteeship Council of the United Nations.

Meanwhile, the U.S. trusteeship bureaucracy continues to function, overseeing the Marshallese government and standing ready to take over again if it fails.

As for what happens to the Kwajalein base should the current rental agreement run out, Muller said he had been told that the Pentagon would return to a prior agreement, which was signed in 1964. It established terms for a 99-year lease, and was signed by Kabua, who in those days was a senator in the Micronesian Congress.