OSLO (Reuters) - Norwegian stock exchange operator Oslo Bors called on Friday for potential buyers to come forward, hoping to find a more generous suitor after Euronext 625 million euro (£562.6 million) offer to buy the company.

Euronext said last week it had secured the backing of a majority of Oslo Bors shareholders, but the Norwegian exchange argued that not all potential buyers had been approached and it would explore alternatives.

"The process will take place in January and the objective will be to find the optimal solution in the best interests both of shareholder value and of the functions of the group's stock exchange and central securities depository activities in the Norwegian securities market," Oslo Bors said in a statement.

Euronext, which operates exchanges in Paris, Brussels, Amsterdam, Lisbon and Dublin, is looking to expand its portfolio but opportunities are scarce as market operators either already belong to groups such as the London Stock Exchange or Nasdaq Inc, or because their shareholders want to remain independent.

Mega-mergers have also met opposition from competition regulators who blocked a planned tie-up between Deutsche Boerse and the London Stock Exchange.

Oslo Bors's board said it intended to meet Euronext and other interested parties before issuing its recommendation to shareholders.

Euronext declined to comment on Oslo Bors's decision to open the process to other bidders but referred to a statement issued last week, implying its original offer remains on the table.

Euronext has said it has the backing of shareholders representing 50.6 percent of the outstanding shares in Oslo Bors. The 625 million euro price tag represents a 21 percent premium over Oslo Bors's market value before the bid was announced.

Euronext said it made its offer within a "compact auction" process. But Olso Bors's Chief Executive Bente Landsnes said last week that potential buyers she did not name had told her they were not invited to bid in the auction.

A deal would match both Euronext's strategy of bolt-on acquisitions and its aim to diversify its revenues from share and derivative trading, with Oslo Bors's leading position in seafood derivatives as well as oil services and shipping.

Euronext owns a 5.1 percent stake in Oslo Bors. Owning more than 10 percent would require approval from the Norwegian government.

While some Norwegian opposition politicians have been sceptical about the deal, junior minister Svein Flaatten has taken a conciliatory tone towards Euronext's move.

(Editing by David Goodman and Susan Fenton)

By Terje Solsvik and Inti Landauro