Suncor Energy Inc. urged Canadian Oil Sands Ltd. shareholders to accept a $4.3-billion hostile takeover as crude prices slump, saying there is little evidence that a rival bid is in the works.

Suncor chief executive officer Steve Williams said the target's management and board members are primarily looking out for themselves in rejecting the unsolicited approach, collecting multimillion-dollar compensation while Canadian Oil Sands shares languish.

Canadian Oil Sands' board and CEO Ryan Kubik together own less than 0.1 per cent of the company's shares, Mr. Williams said in a letter posted to Suncor's website.

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"If they believed their own rhetoric, wouldn't they own more than that? This is the same 10-person group that took home total annual compensation of nearly $5-million in 2014 and oversaw more than $25-million in annual [general and administrative expense] spending in a company with no operations and about 30 people."

Suncor is sharpening its attack in the takeover spat after Alberta's securities regulator gave Canadian Oil Sands until Jan. 8 to drum up a competing bid.

U.S. oil prices have slumped to about $36 (U.S.) in recent weeks, and numerous forecasters are predicting they will stay subdued well past next year.

Mr. Williams said that could force deeper cuts at its partner in the Syncrude Canada Ltd. project. Suncor owns 12 per cent of the venture, while Canadian Oil Sands holds a 37-per-cent stake.

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Suncor is offering 0.25 of one of its shares for each Canadian Oil Sands share and said an outright rejection of the bid could trigger a collapse in the target's stock. It has repeatedly doubted the likelihood of a rival offer emerging and questioned Canadian Oil Sands' ability to continue as an independent entity amid depressed crude prices.

For its part, Canadian Oil Sands has urged shareholders to reject the offer, saying it undervalues the company's operations. It accused Suncor on Tuesday of panicking and trying to distract from a subpar bid.

"If Suncor is unwilling to recognize the significant value of our upgrader, 46-year reserves and 98-per-cent correlation to oil price, shareholders are telling us they see greater value in independence," Mr. Kubik said in a statement. "No amount of fear mongering will change that."

Last week, Canadian Oil Sands chopped its production guidance for the second time this year, citing an advanced maintenance program at its upgrading operations.