Thousands of Vietnamese refugees who recently arrived here are being held in improvised Malaysian government camps and denied United Nations assistance while authorities decide which ones are to be pushed back to sea in repaired boats, reliable sources here said today.

They described this as the government's actual new strategy to curb the flow of refugees, not the much-publicized and roundly denied threat by one official to shoot new refugees and force those already in U.N. camps back to sea.

The Malaysian authorities have expelled thousands in the last few months in old boats patched up at government expense and sent sailing toward Indonesian waters. Between 8,000 and 12,000 refugees are lodged now in makeshift camps on beaches awaiting government decisions on their new move.

The government has prevented U.N. officials from interviewing and registering the refugees, apparently fearing the United Nations would insist they be given asylum in established U.N. camps on the coast. The government is eager to see those official camps emptied and all refugees resettled elsewhere.

Sources familiar with the beach camps say the U.N. representatives here have protested their lack of access but have met with blunt rejections from Malaysian authorities.

They also predicted that in the future far fewer new refugees will find their way to the U.N. sponsored camps and that more and more will be held in limbo for a period of time and then returned to the sea in patched boats.

The strategy in effect shifts a growing share of the refugee burden to neighboring Indonesia. About half of the estimated 30,000 refugees now lodged on the Indonesian Anambas Islands are believed to have come from Malaysian shores in government-provided boats. Indonesia last week announced a new tough policy of its own to turn such boats away.

Sources said the Malaysian system operates seemingly at random, with some refugees being turned over to the United Nations and placed in official camps awaiting resettlement while while others are held for a while in beach areas and then shipped back out. They said the number of new refugees being admitted to U.N. camps dropped sharply early this month although the numbers arriving here from Vietnam has not declined from the large numbers that came in April and May.

Official Malaysian policy always has been to prevent refugees from landing, but in fact authorities had allowed most to come ashore, where they were registered by the United Nations.

Beginning in February, however, Malaysia gradually began shifting to a practice of expulsion. Carpenters were hired to repair the refugee boats, many of which purposely disabled by the refugees. A military task force was formed to round up and guard the newcomers in isolated beach locations.

What is happening now, said one source, "is not a new policy but it is a change in intent."

The government has acknowledged sending 267 boats loaded with some 40,000 refugees out to see so far this year. Many of them had been intercepted before landing, others had been beached temporarily, and the rest had been held for days or weeks in the improvised camps. Food and other care is provided at Malaysian expense, while U.N. funds pay the bill in the established camps.

It is estimated that about 30,000 refugees either reached Malaysian shores or attempted to in May and half of them, according to reliable sources, have already been shipped out.

Local representatives of the U.N. High Commissioner for Refugees protested their lack of access to the government camps. They said they were told they would be kept away by armed guards.

The United Nations is described as being concerned about the seaworthiness of the repaired boats. One capsized on April 4, drowning 104 persons. Some observers believe it was being towed out to sea by Malaysian authorities at the time it sank.

Travelers in waters off Malaysia and Singapore have reported spotting many of the boats, which seemed headed for the Indonesian Islands. One Singapore businessman said he had seen more than 300 refugees sprawling on the deck of an oil rig standing in Indonesian waters. He speculated that their boat had capsized or that they had purposely sunk it to seek refuge on the oil rig.

Indonesia has expanded its navy patrols in an attempt to keep the boats out of its coastal waters. CAPTION: Picture, Vietnamese refugees, their future uncertain, sit in a crowded camp in Kuala Lumpur.