(Recasts, adds analyst comments)
By Nelson Bocanegra
BOGOTA, May 20 (Reuters) - A group of Venezuelan investors are opposed to a bid by Mexico's Alfa and Harbour Energy to purchase oil producer Pacific Rubiales, and said they may continue to buy up more shares of the company, casting uncertainty over the deal.
The investors have increased their participation in Toronto-based Pacific Rubiales over the past two weeks to 19.5 percent from 12 percent and "intend to oppose the acquisition," O'Hara Administration Co said in a statement late on Tuesday on behalf of the group.
Pacific Rubiales, which operates major oil fields in Colombia, said this month that it had received an offer from Alfa and Harbour Energy to buy the company for about C$2.05 billion ($1.68 billion), or C$6.50 per share.
The Venezuelan investor group includes: O'Hara Administration Co; IPC Investments Corp; Telmaven Overseas Inc; Volbor Trading Ltd; Memphis Investment Limited; Agency Partner Corp; Orlando Alvarado; Fundacion Nemone and the Adar Macro Fund.
"O'Hara may take any and all actions it considers advisable, including acquiring additional common shares," the statement said. O'Hara said it does not have plans to make an offer, but "may reconsider doing so if circumstances change."
The purchase offer comes during difficult times for Pacific Rubiales, whose shares plunged 70 percent over the last year amid the global fall in crude prices. The company has already cut investment plans.
News of the bid sent the oil producer's share price up more than 25 percent on the Toronto and Bogota stock markets, but it stabilized as the Venezuelan investors upped their ownership above Alfa's 18.95 percent stake.
The Venezuelan investors did not give details on why they oppose the Alfa and Harbour bid, but analysts said the purchases may be an attempt to pressure the companies to raise their offer or a sign the investors want to launch their own bid.
Alfa and Harbour could call off their bid if they are unable to secure full control, said Sebastian Gallego, oil and gas analyst at Credicorp Capital.
"That's negative for the share," he said. "We consider the offer very good given current risks facing Pacific Rubiales."
Pacific Rubiales shares were up 9.35 percent on the Colombian stock market to 12,860 pesos and rose 7.9 percent to C$6.27 in Toronto on Wednesday.
Alfa has interests in branded foods, aluminum auto parts, petrochemicals and telecommunications services. Harbour Energy is a joint venture between Asian commodity trader Noble Group Ltd and U.S. private-equity firm EIG Global Energy Partners. ($1 = 1.2218 Canadian dollars) (Reporting by Nelson Bocanegra; Writing by Julia Symmes Cobb; Editing by G Crosse and Frances Kerry)