Vietnam demanded yesterday that the United States call off efforts by the 7th Fleet to rescue Vietnamese refugees in the Sougth China Sea, charging the U.S. operation was a "show of force" and that it incited further "illegal departures."

The U.S. government rejected the Vietnamese contention, saying that the ships were on a "humanitarian mission" and that the rescue efforts would continue.

The dispute underscored the extent to which the refugees' plight has become bound up in political issues, often hampnring efforts to rescue and resettle them.

Hanoi's demand came as Typhoon Hoped lashed the Sought China Sea with 150 mph winds and created grave new perils for hundreds, perhaps thousands, of Vietnamese "boat people" adrift in ramshackle vessels.

At least 400 Vietnamese refugees heading for Hong Kong abroad three fishing boats were feared drowned at sea yesturday when they were caught in the storm after being turned away from the nearby Portuguese territory of Macao, according to news agency reports from Hong Kong.

Hong Kong authorities said there was no way of knowing how many more refugee boats were adrift in the Sought China Sea. But they said they doubted that many would survive the typhoon. whose winds destroyed homes and tossed freighters around Hong Kong harhor like toy boats.

Other boats turned away from Macao with 478 Vietnamese refugees aboard had landed safely in Hong Kong before the storm struck, authoreities said.

In denouncing the U.S. rescue efforts, the Vietnamese foreign Ministry accused the 7th Fleet's task force of "inciting and encouraging the ilegal departure of Vietnamese and hindering international efforts to implement" provisions of a July 20-21 Geneva conference on Indochinese refugees.

"This move by the U.S. fleet is also aimed at staging a show of force so as to create tension in Souttheast Asia and jeopardize peace in this region," said the statement broadcast by Hanoi Radio. It said Vietnam "resolutely demands that the U.S. authorities immediately end these activities."

In Washington, a State Department spoknsman said in response that no changes in the rescue effort were planned. He called it a humanitrarian mission representing the best intentions of the American people.

Report from correspondents aboard the aircraft carrier USS Kitty Hawk off Thailand said a five-ship task force of the 7th Fleet left the Thai resort of Pattaya for the South Chinia Sea to conduct a thorough search for more Vietnamese refugees.

The task force is to move around southern Vietnam toward Hong Kong , arriving Friday in areas where boat people in distress are likely to be found, the reports said.

Since President Carter otdered the fleet to search for imperialed refugees on the eve of the Giniva conference, U.S. navel vessels aided by P3 air craft spotting patrols have pocked up 94 boat people, a Pentagon official said. The ships normally picked up only those refugees found in unseaworthy boats. Others are given fod, water and fuel to help them on their way.

Some of the refugees picked up last week during a U.S. naval sweep of 200,000 square miles of ocean told reporters that they had left Vietnam because they had heard foreign radio broadcasts saying the 7th Fleet task force and other ships were in the area searching for foundering boat people.

Dispatching of the new task force to the South China Sea thus raised the prospect of encouraging more Vietnamese refugees to brave stormy weather in the hope of being rescued and eventually resettled in the United States. This would also tend to support Hanoi's charge that Washington is "inciting" refugee departures, regardless of U.S. motives in the mercy mission.

Besides endangering refugees, Typhoon Hope cut a swath of destruction near Hong Kong. Police reported at least four person killed, 260 injured and 2,000 made homeless as fierce winds and torrential rains pounded the coast before the typhoon passed over the British colony and swirled into the southern Chinese mainland.

The winds tore 17 freighters loose from their moorings in Hong Kong harbor and dashed them against peirs, seawalls and other ships. The colony's airport was forced to close.

According to the U.S. State Department, about 400,000 Indochinese refugees are now in U.N. camps scattered around Southeast Asia. An additional 60,000 to 80,000 have not been admitted to the U.n. camps, including about 45,000 Cambodians in Thai camps. The rest are boat people who are living "like shipwreck victims" in squalid conditions and Malaysian and other islands, a State Department official said.

U.S. officials said it was impossible to determine how many lives have been lost at sea. Some estimates have said that around half of those who set out do not survive. The Australian immigration minister has estimated that 200,000 Indochinese refugees have died at sea since boat people began leaving after the Communists took control of Vietnam in 1975.

In addition to braving the elements some refugees have had to contend with attacks by Asian pirates. Those who survived have told harrowing tales of murder, rape and torture on the high seas.

One victim, a 28-year-old Vietnamese woman, told reporters in Thailand today that she was the only one of 100 refugees to survive a recent pirate attack off the southern Thai coast.

She said more than 60 Vietnamese including her husband and two-year-old son drowned when their boat was rammed by pirates in four Thai fishing boats. She said the survivors were then taken aboard the pirates' boats where the women were repeatedly raped and the men beaten with iron bars, clubs and axes.

She said she was spared death by the captain and eventually escaped by going overboard. CAPTION: Map, no caption, By Dave Cook -- The Washington Post; Picture, More than 400 Vietnamese refugees crowd back aboard their boat minutes before being towded out to sea. All were feared drowned in Typhoon Hope. Up1