In a persuasion tit-for-tat, Suncor called on shareholders at Canadian Oil Sands Ltd. to agree to their bid before it expires Friday in order to protect value.

"We are urging Canadian Oil Sands shareholders to act now to protect the value of their investment by tendering their shares to our offer," Steve Williams, Suncor's president and chief executive officer, said in a statement.

Suncor is pushing a hostile bid for Canadian Oil Sands, offering rival shareholders a 45 percent increase in cash dividend. Canadian Oil Sands, for its part, criticized the bid as opportunistic, arguing it's better positioned to navigate the market downturn by itself than Suncor.

Canadian Oil Sands issued what it described as a "declaration of independence" to its shareholders earlier this week. Williams countered in his latest rallying cry that his company was offering a deal that would give rival shareholders a safe haven in the weakened crude oil market.

"That said, we can only invest so much time and money in this effort and will feel compelled to move on to other opportunities if we don't see substantial support for our bid on Friday," he said.

Suncor is a minority stakeholder in the Syncrude oil venture in Alberta. Canadian Oil Sands holds a 37 percent stake in the project.

The consideration comes as this year could bring the conclusion to the merger between Halliburton and oil field services company Baker Hughes and the mega-merger between Royal Dutch Shell and BG Group.