Soviet fishing agreements, rising antinuclear sentiment and muscle-flexing by some tiny island states are forcing the United States to pay closer attention to the South Pacific, a region that U.S. policymakers have long considered "Australia and New Zealand's lake."

The political problems that have arisen in the past few years, according to U.S. officials and diplomats from the region, have allowed Moscow to exploit resentment towards the United States and establish a toehold in a region that has traditionally been pro-West since World War II.

To underscore the growing importance of the region to Washington, Secretary of State George P. Shultz is to visit Palau later this month on his way back from a meeting with foreign ministers of the six-member Association of Southeast Asian Nations. Palau is one of the U.N. Trust territories that voted in February to enter into a relationship with the United States that gives the island group semi-independence and provides Washington with military rights for 50 years.

The recent developments that have drawn U.S. attention include:

*Soviet activity in Kiribati, formerly the British-run Gilbert Islands. The island group, with a land area slightly smaller than New York City, last year signed a fishing agreement with the Soviet Union, the first such agreement by an independent Pacific state. Kiribati has indicated that it would be prepared to talk about another agreement when the current one expires this fall.

*Vanuatu, 80 islands with a land area about the size of Connecticut, is also discussing a fishing agreement with the Soviet Union. In recent weeks, Vanuatu has said repeatedly that it plans to establish diplomatic ties with Libya in return for increased aid and trade. The move drew strong criticism last month from the island's figurehead president, who said it was "causing grave concern" in the region and domestically.

*On May 21, the high chief of Palau filed a lawsuit challenging the validity of the plebiscite that will give the United States military rights on the Pacific island state.

To some extent, the developments are seen as part of the political evolution of the region as the leaders "mature and get more sure of themselves," said one State Department official.

He said, however, that the feeling among some U.S. officials about developments in the past few years is similar to that of "a jilted lover." He added, "the feeling is sort of like, 'How could they do this to us? We were their friends.' "

The concerns of Washington and other states in the region come during a breakdown in the mutual defense alliance between the United States and New Zealand after U.S. Navy ships were effectively blocked from making port calls. The alliance, known as ANZUS, links the United States, New Zealand and Australia and has provided a security umbrella for the region for more than 40 years.

The region is important to the United States not only because of the proximity to sea routes from the U.S. west coast to its trading partners, but also for the U.S. missile test site at Kwajalein in the Marshall Islands.

"What we have here are countries that are not traditionally associated with that part of the world now beginning to be active," said Fiji's ambassador to Washington, Ratu Jone Filipe Radrodro, referring not only to the Soviet Union, but also to Cuba and Vietnam. Vanuatu has diplomatic relations with Cuba and Vietnam.

To change that, Washington is hoping a variety of efforts will boost its profile.

Chief among these efforts is an agreement on tuna fishing, for years one of the most contentious issues between the United States and the Pacific island states. The agreement would allow U.S. tuna boats to fish for tuna in island waters in return for an annual fee. U.S. officials say they hope to reach an agreement this year.

The administration is also debating the merits of signing protocols to a treaty signed by several Pacific nations, including Australia and New Zealand, to make the Pacific a nuclear-free zone. Some U.S. officials are hoping Washington will sign the protocols later this year.

The treaty would allow each South Pacific country to decide whether to allow the transit of nuclear-powered and nuclear-armed warships through the region, a provision that would protect and possibly enhance U.S. interests.

But there is some debate in the Reagan administration that being party to the protocols could fuel antinuclear sentiment in Western Europe and Japan and bring Washington into potential conflict with France, which is strongly criticized in the treaty for its continued nuclear testing in French Polynesia.

A related convention against dumping nuclear waste in the region is also scheduled to be signed later this year by the United States, among others. Here, too, the ban against nuclear testing, adamantly opposed by France, is the one issue that needs to be resolved before the convention can be signed at a diplomatic conference in November, U.S. officials said.

Action by Washington in these areas "would signal to governments that America respects their views," said Kiatro Abisinito, Papua New Guinea's ambassador to Washington. "The United States has taken relations with the Pacific island countries for granted for too long."

Fishing rights have caused by far the greatest friction. The United States does not recognize the island states' exclusive right to fish tuna that swim in the 200 miles around their coasts. As a result, U.S. tuna boats have refused to pay fishing fees to the islanders, who rely on income derived from fishing.

Two years ago, a U.S. tuna boat, the Jeanette Diana, was seized by the Solomon Islands on charges of poaching, triggering a U.S. embargo on all tuna from the Solomon Islands.

"Throughout the region we were cast as the bully beating up on the small defenseless countries that had only one thing in the world," said one State Department official who was stationed in the region at the time. "The [U.S. tuna] boats had moved from poachers to pirates . . . It was a terrible political problem."

The incident prompted the United States to renew efforts, begun in 1983, to negotiate a regional agreement.

Representatives from Washington and the islanders are to meet next month in the Cook Islands for another round of talks on one of the few outstanding issues, a fee.

In the case of Kiribati, President Ieremia Tabaihas said his country's fishing agreement with the Soviet Union is strictly a "commercial deal" for the island, which has a population of about 56,000 and a budget of about $18 million. Moscow sought but was denied shore rights.

But in the view of most analysts, $1.5 million to allow 16 Soviet vessels to fish in Kiribati's waters was too much money for the value of the fish. "There is a political component in the fee, and clearly, in our assessment, the Soviets are trying to establish a presence," a State Department official said.

Moscow's efforts have met with limited success. Despite similar offers to other island nations, Kiribati has been the only one to accept so far. There is still a reservoir of goodwill toward the United States left over from its role as a liberator and protector after World War II, U.S. officials and diplomats say.

But the U.S. concern is how long the small countries will be able to hold out.

"Russian attempts to penetrate the South Pacific are becoming more sophisticasted," said Adm. James Lyons, commander of the U.S. Pacific Fleet, during a visit to Fiji last month.