The United States has told the Republic of the Marshall Islands government that it wants to delay an Aug. 17 plebiscite by which the Pacific islanders were to vote on a compact of association between the two nations.

The U.S. decision was transmitted last week in a letter from Undersecretary of State James L. Buckley to Marshall Islands President Amata Kabua. It came as a "surprise" to the Marshallese, according to James Boyle, a lawyer in Hawaii who has represented the islands' government in compact negotiations this year.

When Kabua returned to the Marshalls from Hawaii, where the compact was signed, he announced with much publicity that the plebiscite would take place Aug. 17.

The compact, signed in May, includes two articles that have proved controversial to some Marshallese: a $100 million settlement of radiation claims arising from American nuclear testing in the Marshalls and a 50-year rental of Kwajalein Atoll for U.S. strategic missile testing.

A State Department official said the requested delay in the plebiscite "was no big thing and has nothing to do with opposition to the agreement." But a Washington lawyer with islander clients said U.S. officials were "worried" about the election. He cited the resignation of Marshallese Minister of Internal Security Ataji Balos so that he could work against the compact. Balos is chairman of the board of the Kwajalein Atoll Corp., whose members are landowners who believe the proposed $9 million-a-year rent for their islands is not enough to support the more than 6,000 people living there.

With the knowledge that the Pentagon had planned to test fire a Minuteman missile from California with the warhead landing in Kwajalein lagoon in the next few days, Balos led a group of Marshallese protesters onto two islands in the atoll where monitoring and radar equipment operate. Both areas are out of range of the lagoon target area.

The Marshallese government obtained a court order that the demonstration be halted but, according to a Pentagon official, the protest continues.

Another concern, according to sources, is the ballot being proposed by the Kabua government. It would require voters to pick between two choices: the compact and full independence. Some Pentagon officials fear that should the compact lose, the Kwajalein missile test site might be lost also under the "full independence" concept.

They want the voters to have a third choice, continued trusteeship, which the Marshall Islands government strongly opposes.

The Buckley letter, according to a State Department official, says that the August date was too early to permit voter education programs, allocation of radio time for compact opponents and organization of a U.N. observer team. The official pointed out that under terms of the compact the United States is to set the date for the plebiscite, timing it with elections in two other trust territories, Palau and the Federation of Micronesia.

Boyle said Kabua and other island officials "thought they had an agreement" on the August date because of a May 16 memorandum of understanding between the chief U.S. negotiator, Ambassador Fred M. Zeter, and Tony A. DeBrum, foreign secretary of the Marshall Islands government.

It said that each would use his "best efforts" to hold the election on Aug. 17 and, based on that document, the Marshallese were carrying out their voter education program.

Kabua is traveling in Japan, Boyle said, and would not have an immediate response. He added that a meeting between Zeter and DeBrum was scheduled for next month and a new election date may be set then.