MONTREAL (AFP) - A Russian container ship carrying tons of fuel and diesel was drifting once again off Canada's Pacific shore after a cable linking it to a tow ship ruptured, the military said.

The ship is carrying 400 metric tonnes of bunker fuel and 60 metric tonnes of diesel, triggering fears of an environmental catastrophe.

A Canadian Coast Guard boat had managed to tow the cargo ship about 30 kilometres toward the open sea since late Friday, but the cable linking the two vessels ruptured for the third time late Saturday.

"It's not unusual" given the rough seas in the area, Royal Canadian Navy Lieutenant Greg Menzies told AFP.

Two other Canadian Coast Guard ships are traveling alongside the Simushir.

A powerful tow boat was expected to assist in the efforts later in the day. "We still have other assets," Menzies said.

The Russian ship had been heading home from the US state of Washington when it suddenly lost power in bad weather overnight Thursday and became stranded eight nautical miles from the west coast of the Haida Gwaii archipelago.

Local aboriginal people had declared a state of emergency, fearing the ship would strike land and devastate the pristine coastline, which is home to a rich marine life and dozens of ancient village sites.