(Bloomberg) -- Anadarko Petroleum Corp.’s board has declared Occidental Petroleum Corp.’s sweetened $38 billion takeover “superior,” allowing Chevron Corp. to either boost its offer or walk away with a $1 billion breakup fee.

The announcement by Anadarko, which had rebuffed several prior bids from Occidental, comes almost a month after it agreed to be purchased by Chevron for $33 billion. Chevron has so far refused to increase its offer, saying its proposal is a better long-term option for Anadarko shareholders.

Chevron now has four days to revise its bid or submit a new offer, though that may be extended under the terms of the original agreement, Anadarko said in a statement after the market closed Monday. Chevron declined to comment.

The takeover of Anadarko has been a rare public fight for an oil industry that hasn’t seen many bidding wars. Occidental Chief Executive Officer Vicki Hollub’s recruitment of billionaire Warren Buffett and an $8.8 billion accord to offload African oil and gas fields to Total SA appeared to have been key milestones in winning the board’s favor. Anadarko directors had previously chosen Chevron’s offer even though it was substantially cheaper than that of Occidental, which, as a smaller company, would have to take on much more leverage to do the deal.

Chevron CEO Mike Wirth has signaled he’s willing to end his pursuit of a deal if the price gets so high that it imperils investor returns. “We strongly believe that the combination of our two companies create superior long-term value for shareholders of the combined company,” Wirth said on an April 26 earnings conference call.

While buying Anadarko would expand the producer’s presence in the Permian Basin, which is now the world’s largest oil patch, there are other operators in the region that Chevron could acquire. Chevron also has existing ambitious growth plans for the Permian.

Houston-based Occidental over the weekend pressed to alleviate any remaining concerns from Anadarko. It said Sunday that 78 percent of the deal will be paid in cash, up from 50 percent, and it eliminated the need for a shareholder vote, which had previously been cited as a sticking point for Anadarko’s directors.

“With the OXY deal looking safer for APC to accept, I’d say this may force CVX to match the OXY bid or lose out on these terrific assets,” said Bill Nygren, chief investment officer of Harris Associates LP, which manages $120 billion and owns about 3 percent of Anadarko.

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