U.N. Secretary General Javier Perez de Cuellar arrived here today to discuss with Vietnamese leaders the prospects of a settlement in Cambodia, where Vietnamese troops are waging a dry-season offensive against Cambodian guerrillas.

The U.N. chief immediately began a round of talks with Vietnamese Foreign Minister Nguyen Co Thach. But the prospects for progress in settling the six-year-old conflict in Cambodia appeared remote, diplomats said.

The visit coincided with an upsurge of fighting between Vietnamese forces and Cambodian resistance groups along the Thai-Cambodian border. According to reports from the border, about 20,000 persons fled toward Thailand but were turned back by Thai forces after fierce fighting erupted between Vietnamese troops and Khmer Rouge guerrillas.

Thach told reporters before Perez de Cuellar's arrival that the United Nations had "bloody hands" for recognizing as Cambodia's legal government a resistance coalition that includes the brutal Khmer Rouge. Under dictator Pol Pot the communist Khmer Rouge brought widespread destruction and death to Cambodia during nearly four years of rule before Vietnamese invasion forces swept them from power in January 1979.

Thach also charged that U.N. organizations were allowing Cambodian refugee camps along the Thai-Cambodian border to be used as "a facade for covering crimes" against the embattled country.

In a somewhat more conciliatory tone, Thach said the United States could contribute to a political solution in Cambodia, but did not specify how or what kind of solution he envisaged. He said that "it would be very logical" for the United States and Vietnam to pursue joint efforts for peace in Southeast Asia since both have influence in the region.

Thach is scheduled to confer with Perez de Cuellar in a second round of talks Tuesday. The U.N. chief then is to meet Prime Minister Pham Van Dong. Perez de Cuellar ends his visit Wednesday.

On his way to Hanoi from Thailand, Perez de Cuellar stopped briefly in Laos, where he raised the issue of American servicemen missing in action in the Vietnam war, a spokesman said.

The spokesman, Jean-Francois Giuliani, said the issue had been put on the U.N. chief's agenda after talks in New York with U.S. Ambassador Jeane J. Kirkpatrick. About 560 American servicemen who disappeared in Laos during the Vietnam conflict remain unaccounted for.

Perez de Cuellar raised the issue with Laotian Vice Premier Phoun Sipaseuth, Giuliani said, but he gave no details of the Laotian response.

The secretary general also carried a message from the Thai government to the communist administration in Laos on improving strained relations, and was given a Laotian message to take back to Thailand.