The two principal owners of United Press International are attempting to sell their stock in the troubled wire service, one of the owners said yesterday.

But their attempts are being opposed by top management of the company, which claims that such a sale would undermine efforts to recapitalize the troubled news organization.

Meanwhile, the Wire Service Guild, which represents UPI's employes, has told the company it will accept no more contract concessions without sufficient cause, the union said in a bulletin to its members yesterday. The guild accepted wage cuts as deep as 25 percent and made other concessions last year as part of a plan to return UPI to profitability after 20 years of operating in the red.

The disagreement between UPI's owners and managers was characterized in yesterday's guild bulletin as a "power struggle," but it is being downplayed by UPI's management. UPI spokesman David Wickenden said yesterday the disagreement was "minor stuff."

But at the very least, the dispute appears to threaten the fragile peace that has existed at the top of UPI since a bizarre upheaval last month that saw owners Douglas Ruhe and William Geissler fire president Luis Nogales and then reinstate him the next day, along with several other officers who had quit in protest, under pressure from the company's lenders.

At that time, Ruhe and Geissler entered into an agreement to give up their 83 percent ownership in the company under a recapitalization plan that, according to one scenario, might include the issuance of UPI stock to the union, management and creditors, or all three. Such a plan would have to be approved by UPI's new board, which includes Ruhe, Nogales, Wire Service Guild President Bill Morrissey and UPI Editor-in-Chief Maxwell McCrohon.

Ruhe said he and Geissler have begun to take steps to offer their stock for sale once the recapitalization program is in place and the agreement with the company is terminated. He said the attempts are being blocked by Nogales, who claims the agreement prevents Ruhe and Geissler from selling their stock without the board's assent.

"We are offering to sell our shares upon termination of the agreement," Ruhe said in a telephone interview from his Nashville office. "They say that we don't have that right."

Nogales could not be reached for comment. Wickenden declined to discuss the agreement between UPI and its owners, but said a statement from Nogales on the matter could come within the next couple of days.

Ruhe said he did not see how Nogales could be preparing to say anything definitive, because the two sides have been unable to negotiate an agreement on the dispute. "He was totally unwilling to consider any kind of cooperation or common effort," Ruhe said of Nogales. The guild bulletin yesterday said the union was attempting to intercede to bring about a solution.

Ruhe and Geissler's agreement with UPI does not include any kind of arbitration clause, according to Ruhe, and he said Nogales has rejected a suggestion that Ruhe, Nogales and either McCrohon or Morrissey form a committee to look into the sale of the stock. The alternative may be legal action, which Ruhe said he did not want. "We don't do business by litigation," he said.

It is unclear how the dispute will affect UPI's attempts to recapitalize and return to financial health. The wire service has lurched from one crisis to another in recent months, each time appearing to reach a settlement that would allow it to survive for a long period of time, only to have new problems a few weeks later.

Ruhe and Geissler purchased UPI in 1982 from Scripps-Howard, which had operated it at a loss for nearly 20 years. Reportedly, the two Nashville businessmen acquired the wire service for virtually nothing but an assumption of its debts, which continued to mount.

Ruhe would not say yesterday whether he and Geissler had gotten any indications that there might be potential buyers for their stock, although Australian press magnate Rupert Murdoch is known to have turned down an offer to buy UPI last month.

"We're trying to find a purchaser of our 83 percent interest that would have the wherewithal, the financial strength and the commitment to ensure the financial strength and health of UPI," Ruhe said. He said it was "hard to say" what the two owners' stock is worth, but he noted that the company was valued at $40 million when former Treasury secretary G. William Miller, acting on Ruhe and Geissler's behalf, attempted unsuccessfully to find new investors for the wire service last year.