Spear-carrying warriors in grass skirts, women wearing sarongs and singing island songs and an urbane Oxford-educated prime minister in coat, tie and skirt welcomed Secretary of State George P. Shultz back to the South Pacific here today.

Shultz, who spent two years in the area as a marine lieutenant fighting the Japanese in World War II, was welcomed with colorful Fijian ceremony as the highest ranking American to visit since the island nation won its independence from Britain in 1970.

Shultz's four-hour stopover, en route to Hawaii from an official visit to Southeast Asia and Australia, symbolized growing U.S. interest in the island nations that dot the vast blue waters of the South Pacific.

Fiji is a likely place for the United States to start: it is one of the most populous of the island groups -- with 670,000 people -- and one of the most constructively involved with the rest of the world. This pocket-sized nation has contributed a battalion of troops to the U.N. peace-keeping force in southern Lebanon and another battalion to the U.S.-sponsored peace-keeping force in the Sinai.

Shultz arrived here fresh from dealing in Australia with the consequences of New Zealand's refusal to accept port calls of U.S. warships. But he noted that Fiji, while not bound to the United States by any alliance, had recently taken the "bold and wise" step of permitting U.S. Navy port calls. Moreover, although Shultz did not mention it, Fiji's leadership has consistently rejected requests from the Soviet Union for a permanent diplomatic presence, and Soviet cruise ships have been refused access to Fijian ports.

One reason for Fiji's frigid attitude toward the Soviets, according to U.S. sources, is that Prime Minister Ratu Sir Kamisese Mara suspects Moscow of meddling in local politics by helping to finance his opponents in the last elections.

Mara, addressing the welcoming ceremony just after he and Shultz had downed coconut shell cups of the potent island liquor, yaqona, declared that Fiji is "fully committed to a policy of seeking the closest possible relations with the United States."

Mara added, "We look to your country as a friend of all in the South Pacific." He visited President Reagan in Washington late last year. Since then, the United States has agreed to landing rights for Fiji's aircraft in Los Angeles and has asked Congress to begin an economic aid program of $1.5 million.

In addition, Shultz issued orders to U.S. Ambassador Edward Dillery to work with the U.S. Food and Drug Administration and other agencies to permit the importation of yaqona, which so far has been barred from the United States as a dangerous drug.

Fiji is believed to take a dim view of the decision by Kiribati, the former Gilbert Islands, to permit a Soviet fishing fleet to operate in its waters in return for a reported $1.5 million annual fee. Shultz has expressed concern about the deal, citing it as an example of an unwelcome Soviet presence.

"Fiji has a big influence among the Pacific island nations," said an aide to Shultz in explaining the attention being paid to it by the United States. Fiji's voice is particularly important in the South Pacific Forum, an association of the island states and such larger nearby powers as Australia and New Zealand.

Beyond the Fiji connection symbolized by today's visit, the Reagan administration wants to improve its overall relations with Pacific island nations. Part of this effort is the work being done on a fishing agreement that would regulate the problem of fishing rights and diminish conflicts with island nations over the activities of U.S. tuna boats.