A Carter administration plan to temporarily extend the attorney general's "parole" program to allow more refugees into the country is drawing fire on Capitol Hill.

The attack is coming from lawmakers who say some of the nation's staunchest Indochinese allies, the Hmong hill tribesmen of Laos, are being left to fend for themselves in Southeast Asia.

Since 1975, the United States has admitted 223,000 Indochinese refugees, 17,000 of them Hmong tribesmen. Most of the Indochinese were admitted on the attorney general's parole authority. Ordinarily, they would have been barred because their numbers exceeded the annual statutory "normal flow" ceiling of 17,400 admitted to the United States.

The attorney general's current parole program expires April 30. President Carter and Secretary of State Cyrus R. Vance want to extend it to Sept. 30. Therein lies a major part of the problem.

The proposed extension would permit 35,000 more Indochines at the rate of 7,000 a month to enter. Congressional sources say the monthly influx is broken down into 5,000 "boat people," many of them ethnic Chinese escaping from Vietnam aboard an asescaping of vessels, and 2,000 "land people," persons already in refugee camps and those, like the Hmong, who escaped on foot.

"The Hmong tribesmen are leaving Laos because a campaign of terror is being waged against them by (Communist) Vietnamese and Laotian forces," said Rep. Matthew F. McHugh (D-N.Y.), a member of the House foreign operations subcommittee.

McHugh said the Hmong are targets in Southeast Asia because they worked closely with the Central Intelligence Agency in Laos and "fought bravely and valiantly for the United States throughout our involvement" there.

But, instead of fulfilling that obligation, the United States would admit only about 800 Hmong tribesmen a month under the extended parole program, according to McHugh.

"That's a very inadequate number when you consider that the Hmong are fleeing Laos at the rate of between 2,000 and 2,500 a month," McHugh said. To do better by the Hmong and other "land people," the United States should increase the monthly slots in its extended parole program to 3000, he said.

Other Capitol Hill sources complained that the administration has decided to place any type of ceiling on Indochinese admissions in the extended program, which does not need congressional approval.

"We've been arguing for greater flexibility of parole numbers," one source said. "We shouldn't arbitrarily freeze these numbers.... Under these circumstances, you need the wisdom od Solomon to determine who's going to come and who's going to stay behind," he said.

Frank Sieverts, the State Department's deputy assistant secretary for refugee and migration affairs, said yesterday the administration has no intention of slighting the Hmong tribesmen or any other Indochinese refugees in its selection process.

However, the State Department official conceded that the administration has recently placed high priority on saving the so-called boat people "because of their situation, because they are in danger of immediate death and of being thrown back into the sea."

Sieverts said the administration hopes Congress will help to alleviate much of the agony of deciding who comes and who stay by acting quickly on legislation to overhaul U.S. refugee laws and to raise the statutory "normal flow" ceiling from 17,400 to 50,000 annually.