An offer by a group of unidentified investors to buy United Press International expired yesterday as the bidders refused to give the financially strapped news service additional time to consider their $17 million proposal.

"We're very disappointed. Our offer was on the table for two weeks. We just can't keep an offer on the table forever," said David Rubenstein, a Washington attorney representing the investors.

UPI is seeking a buyer to help take it out of bankruptcy court. In the interim, the 75-year-old news service is protected from its creditors as it tries to reorganize under Chapter 11 of the federal bankruptcy code.

UPI had assets of $20 million and liabilities of $45 million when it filed for protection late in April.

Rubenstein said his investors were taken aback by UPI's apparent willingness to "shop around" for other offers before considering their proposal in depth.

"We didn't want to be shopped," he said. "UPI wanted another month to review our offer while they were looking at other offers," Rubenstein said.

The lawyer said that his investors' decision to let their offer expire does not necessarily mean that they have lost all interest in UPI.

Nor does it mean that they will renew the defunct bid, or come up with a replacement offer, Rubenstein said. "We'll have to see what happens in the future," he said.

William Adler, a UPI spokesman, said yesterday that the company allowed the offer to die because two weeks was not enough time to give the bid proper consideration.

"That's the sort of way it's been, so far, with offers that have a short deadline," Adler said.

"Offers with short deadlines don't work too well in this system because we have so many groups that have to look at it" before the company can act on a bid, Adler said.

A UPI official said the company is considering other offers, which he declined to discuss.