Gangs of Vietnamese refugees in 13 states are expanding criminal operations that include murder, extortion, gambling, prostitution, narcotics, robbery and smuggling, the President's Commission on Organized Crime said today.

An unidentified witness, claiming to be a high-level operative in an unidentified Vietnamese gang, testified that former South Vietnamese prime minister Nguyen Cao Ky, a resident of Orange County, Calif., masterminds four gangs in Los Angeles, San Francisco, Houston and Chicago with the aid of four or five former South Vietnamese generals.

The witness, brought to the hearing disguised in black robes and a hood, testified behind a screen. He said he is of Vietnamese-Chinese origin, came to the United States in 1975 and was trained in Los Angeles to handle machine guns and commit crimes.

Police officials from Houston, Orange County, New Orleans and Arlington, Va., which have the nation's largest concentrations of Vietnamese, said in testimony and interviews that the witness' allegations about Ky are uncorroborated.

Police Capt. Donald Saviers of Westminster, Calif., which, with neighboring Orange County towns, has about 90,000 Vietnamese residents, said the allegations surfaced in 1979 in a "raw intelligence" report by an Arizona narcotics task force.

Ky, who owns a liquor store and is active with Vietnamese refugee groups, vigorously denied the report.

The witness said Ky's organization is known as "the dark side or the black side" of an above-ground anti-communist group. The gangs it controls, he said, are "Black Eagle" in San Francisco, "Fishermen" in Houston, "Eagle Seven" in Chicago and "Frogmen" in Orange County.

Houston police officer Robert E. Casey Jr., in charge of investigating crimes involving Vietnamese there, testified, "I have several disagreements with that witness. In eight months of full-time investigation, I have never heard of the 'Fishermen.' "

Casey said two Vietnamese gangs have been active in Houston but are crippled by recent prosecutions and deaths. Twelve gang-perpetrated homicides are unsolved there, and extortion of Vietnamese merchants for protection money is a continuing problem, according to Houston police.

Commission spokesman Arthur P. Brill declined to say whether its investigators have any reason to believe allegations about Ky. "We can't comment on what comes out in testimony," he said.

Noting that no police departments have Vietnamese-speaking officers, he said, "Local police officers do not have a handle on the hierarchy in their own areas or nationally."

Arlington Police Det. James R. Badey said in prepared testimony that Washington-area Vietnamese have "loose-knit groups attempting to assert themselves as the 'main gang' . . . . There have been many instances throughout the metropolitan area involving Vietnamese as the victims of arson, assault, burglary, extortion, murder and robbery."

Badey said he has "personal knowledge of major frauds, major gambling operations, loan-sharking, narcotics activities and prostitution . . . . We have corroborated information that Vietnamese have gone to New York to join the New York Chinese street gangs to gain experience . . . ."

"If the Vietnamese criminal element is allowed to continue unchecked or only moderately checked we shall have, in the not too distant future, a well-defined highly sophisticated, organized crime machine which when compared to the Mafiosa sic will make the Mafiosa sic look like a fraternity of wimps," Brady's written testimony said.

The commission also heard testimony from the widow of Dam Phong, publisher of Tu Do magazine of Houston, who was shot to death in August 1982 after publishing articles about extortion schemes by Houston gang members.

Next to Phong's body, Houston police found a document stating in Vietnamese that the "Vietnamese Party for the Annihilation of Communism and for the National Restoration" had made a "judgment" on April 30, 1982, against Phong and four other Vietnamese journalists and calling them "the most dangerous Vietnamese Traitors."

One of those cited was Nguyen Than Hoang, publisher of Van Nghe Tien Phong magazine in Arlington. His home was firebombed in 1980 after he received threats about his articles. Another California journalist survived a firebombing, and several others have been threatened.

But Houston Police Sgt. Jerry Ellis said the FBI and local police have been unable to determine whether the document and the organization are "real or phony."

Besides those mentioned, states cited by the commission are Massachusetts, New York, Pennsylvania, Alabama, Florida, Oregon, Hawaii, Washington and Colorado.