Australian publisher Rupert Murdoch, whose media holdings span three continents, told the Federal Communications Commission yesterday he plans to become a U.S. citizen to allow him to buy six television stations from Metromedia, according to commission sources.

FCC officials said Murdoch also left them with the impression he would be willing to sell the New York Post and Chicago Sun-Times, if necessary, to acquire the Metromedia stations in those cities.

Commission rules prohibit foreign citizens from owning more than 20 percent of a broadcast station and 25 percent of a company that owns a broadcast station. The rules also prohibit individuals or companies from owning newspapers and television or radio stations in the same market.

The bid for the Metromedia stations is being made through an entity controlled by Twentieth Century Fox Film Corp. Twentieth Century is owned by Murdoch and Denver oilman Marvin Davis.

"He told me he plans to become an American citizen," said Commissioner James H. Quello after meeting yesterday with Murdoch and John W. Kluge, chairman of Metromedia. "And he is going to meet all FCC requirements -- there are no indications of any waivers he might be requesting."

"I would assume the newspapers are going to go," Quello said. Murdoch could not be reached yesterday for comment.

Murdoch is discussing the acquisition of Metromedia stations in New York, Los Angeles, Chicago, Houston, Dallas and Washington. This would possibly require the sale of the New York Post, which reportedly had been losing between $15 million to $20 million a year since Murdoch acquired it in 1977. It was unclear whether FCC rules would require Murdoch to sell the Village Voice, New York Magazine, both of which are profitable.

Murdoch acquired the Sun-Times more than a year ago in a $90 million purchase from Field Enterprises. The Sun-Times is reportedly running in the black. Metromedia owns a UHF broadcast station in Chicago.

Murdoch also owns the Boston Herald, where Metromedia also owns a television station. But Metromedia reportedly does not plan to sell the station to Murdoch so there would be no cross-ownership conflict.

Stanton Cook, chairman of the board of the Tribune Co., which competes against Murdoch with newspapers in both New York and Chicago, would not comment on the proposed Metromedia sale to Murdoch. Cook said he would comment "when it's the right time and this isn't the right time."

Murdoch and Kluge met with FCC Chairman Mark S. Fowler on Tuesday and met yesterday with the other four commissioners, Quello said. "I said 'Mr. Murdoch, how do you plan to serve the public interest?' And he said 'I have previous experience in Australia and considerable capital I can bring to broadcast programming and production.' "

Quello said as that as long as Murdoch complied with all FCC requirements, including citizenship, and filled out a "long form," which allows a 30-day comment period on the transfer of broadcast licenses, he saw no "bar to his acquiring the stations."

The proposed acquistion may also pose problems for Murdoch overseas. He currently owns a television network in Australia as well as newspapers. According to sources at the U.S. Immigration and Naturalization Service, Murdoch would have to renounce his Australian citizenship to become an American citizen. Consequently, the Australian broadcast commission may have to review his station holdings in that country unless Australia has a provision for dual citizenship.

Murdoch has even more extensive holdings in the United Kingdom. He owns the Sun, which has the largest circulation in England, as well as the Times of London. He is also planning to launch a new afternoon paper. In the past, the Newspaper Monopoly Commission, which reviews newspaper ownership in Britain, has expressed concern over foreign ownership. This concern was specifically expressed when Atlantic Ritchfield Corp. made a bid to acquire the London Observer.