Canadian Oil Sands Ltd., in the latest in a bitter dispute, urged shareholders to take no action on a revised takeover bid from rival Suncor Energy.

Suncor announced it filed an extension to the expiration of a bid for Canadian Oil Sands, its co-venture partner at Canada's mega Syncrude oil sands production facility, from Friday to Jan. 8. The company has made a full-stock offer that President and Chief Executive Officer Steve Williams said is "full and fair."

Canadian Oil Sands directors have been firm in their stance on the hostile takeover bid.

"Extending the expiry of Suncor's bid does not change the fact that it is substantially undervalued and opportunistic," Donald Lowry, chairman of Canadian Oil Sands, said in a statement. "Nothing else has changed so, as it stands, there is more value for shareholders in a strong, independent company than there is in this offer."

Suncor is a minority stakeholder in the Syncrude oil venture in Alberta. Canadian Oil Sands holds a 37 percent stake in the project. Suncor warned rival shareholders of the risks they face if they reject the offer in a weakened crude oil market. Several major oil and oil services company are in the process of merging in an effort to streamline operations.

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.

Canadian Oil Sands counters the Suncor bid is an opportunistic move by a minority stakeholder.

"Suncor wants the company because Canadian Oil Sands is more highly leveraged to oil prices than Suncor," it said.

When energy stocks rose as a whole, the company added its shares gained more than twice as much as Suncor's.

Lowry earlier this week noted the company's board was examining "superior offers."