The United States will support the seating of Pol Pot's "democratic Kampuchea" regime in the United Nations again this year despite its abhorrent record on human rights, Secretary of State Edmund S. Muskie announced yesterday.

Speaking to a news conference, Muskie said the U.S. decision -- the subject of speculation and controversy at home and abroad -- was made at the behest of Southeast Asian allies and after "careful diplomatic soundings" that Vietnam is unwilling to negotiate the withdrawal of its forces from Kampuchea.

A credentials challenge to "Democratic Kampuchea," which currently occupies the U.N. seat, is expected in the early days of the General Assembly session, which begins in New York today. The challenge will be mounted by Vietnam and the "People's Republic of Kampuchea", which is ruling most of Cambodia (Kampuchea) from Phnom Penh under Vietnamese sponsorship.

China and Japan, as well as U.S. allies in Southeast Asia, have argued that to unseat the Khmer insurgents, whose military leader is Pol Pot, would be to give international legitimacy to the Vietnamese occupation of Cambodia.

State Department officials said a major factor in the U.S. decision was an inflexible Vietnamese stand regarding withdrawal from Cambodia, taken in unannounced talks last month between Vietnamese officials at the United Nations and Deputy Assistant Secretary of State John D. Negroponte. The decision on the U.S. position was made at high levels of the government toward the end of last week, officials said.

Muskie said the U.S. decision "in no way implies any support or recognition of the Democratic Kampuchea regime. We abhor and condemn the regime's human rights record and would never support its return to power in Phnom Penh."

He went on to say that the U.S. vote on the contested United Nations seat "can prevent legitimization of a government installed by aggression and maintained by the presence of any invading army."

In Portland, Ore., independent presidential candidate John B. Anderson charged that President Carter is violating his own commitment to human rights in supporting the seating of the Pol Pot group.

"Many of us frankly were dismayed last September when the U.S. representative to the United Nations voted in support of seating the Pol Pot regime, a brutal gang of terrorists whose misrule cost the lives of between 1 million and 2 million Cambodians," Anderson said. He charged that continued U.S. support for that position is a "striking contradiction of the Carter administration's human rights policy."

Anderson called for the United States to vote against both Cambodian claimants to the U.N. seat, leaving it vacant until a legitimate representative can be found.

Ten U.S. senators have signed a letter to Muskie, sponsored by Sens. Carl Levin (D-Mich.) and Paul E. Tsongas (D-mass.), calling for the United States to abstain on the vote for the Cambodian seat at the United Nations. "We should stand apart from both the brutality of the communist Pol Pot government and the communist, Vietnamese client-government of Heng Samrin," the letter said.

Muskie, in his news conference the State Department, also said:

He expects to reach agreement with Soviet Foreign Minister Andrei A. Gromyko in New York late next week on the time and place of preliminary discussions between the two nations on the limitation of medium-range nuclear missiles in Europe.

Muskie's confidence, according to administration sources, grows out of his unannounced meeting at the State Department last Saturday with Soviet Ambassador Anatoliy F. Dobrynin, at which the United States proposed that the talks begin in the second week of October in Geneva at the level of senior experts. Dobrynin did not accept the U.S. plan as presented, but apparently was encouraging in his position.

It is "a fair assumption" that India will consider its 1963 nuclear agreement with the United States to be void of the United States refuses to ship 38 tons of nuclear fuel that India has ordered. The consequences would include Indian removal of previously supplied U.S. fuel from international enspection, "a precedent . . . that would strike at the very heart of our efforts to halt the spread of nuclear weapons."

Senate and House committees on foreign relations voted last Wednesday to stop shipment of the disputed fuel. A final decision by both houses is expected soon.