In a declaration of independence, Canadian Oil Sands Ltd. urged shareholders to reject a hostile bid from rival Suncor before the offer expires on Friday.

"Suncor's substantially undervalued bid is set to lapse, and when it does they say they will walk away," Canadian Oil Sands board Chairman Don Lowry said in a statement. "For all of us, as shareholders, this scenario reveals a far more compelling and valuable alternative: Independence."

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

Suncor is making a hostile bid for Canadian Oil Sands. Suncor President and Chief Executive Officer Steve Williams said the offer is "fair."

Canadian Oil Sands criticized the bid as opportunistic, arguing it's better positioned to navigate the market downturn by itself than Suncor. Suncor, meanwhile, said it could help return value to shareholders.

"We can cover operating and capital costs and pay the dividend in a low oil price environment," Lowry said.

Suncor said rival shareholders would receive a 45 percent cash dividend increase. Over a five-year period ending in October, the company added its divided increased 190 percent while dividends for Canadian Oil Sands shareholders fell by 90 percent.