(Adds Pacific Rubiales' option to consider other offers, adds quotes and background)
BOGOTA, May 21 (Reuters) - Pacific Rubiales Energy Corp , the largest private oil producer in Colombia, has accepted a C$6.50-per-share takeover bid from Mexican conglomerate Alfa SAB de CV and investment firm Harbour Energy Ltd but said it might consider other offers.
The announcement came late on Wednesday after a group of Venezuelan investors opposed to the Alfa-Harbour bid said they might continue to buy up more Pacific shares.
The Venezuelan group has raised its stake in Toronto-based Pacific to 19.5 percent from 12 percent over the past two weeks.
Pacific said this month that it had received an offer from Alfa and Harbour Energy to buy the company for about C$2.05 billion ($1.70 billion), or C$6.50 per share.
Pacific shares were up 2.58 percent at C$6.35 on Thursday.
The news of the bid had sent the shares up more than 25 percent on the Toronto and Bogota stock markets earlier this month, but they stabilized as the Venezuelan investors increased their ownership above Alfa's current 18.95 percent.
Pacific's board unanimously approved the Alfa-Harbour offer, it said in Wednesday's statement, and the company expects the deal to close in the third quarter, pending shareholder approval.
But it said it could accept any "superior proposal" to acquire all outstanding common shares if Alfa and Harbour do not match it.
Alfa and Harbour would each be entitled to a $50 million termination payment if Pacific accepts a superior proposal.
The Venezuelan investors did not say why they oppose the Alfa-Harbour bid. Analysts said the share purchases might be an attempt to pressure the companies to raise their offer or a sign the investors want to launch their own bid.
O'Hara Administration Co, speaking on behalf of the group, said it had no plans to make an offer but might reconsider if circumstances change.
Besides O'Hara, the investor group includes IPC Investments Corp; Telmaven Overseas Inc; Volbor Trading Ltd; Memphis Investment Limited; Agency Partner Corp; Orlando Alvarado; Fundacion Nemone; and the Adar Macro Fund.
The purchase offer comes during difficult times for Pacific, whose shares plunged 70 percent over the last year amid the global fall in crude prices. The company has already cut investment plans.
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. (Reporting by Nelson Bocanegra and Julia Symmes Cobb in Bogota and Rishika Sadam in Bengaluru; Editing by Gopakumar Warrier and Lisa Von Ahn)