The stricken container ship, Rena, has started to break apart off the coast of New Zealand

A cargo ship grounded off the New Zealand coast since October has split into two pieces after being lashed by pounding seas, spilling sea containers and debris and sparking fears a fresh oil spill could wash ashore, maritime officials said on Sunday.

The officials said that the front section of the wreck remains stuck in its original position, but the stern section has broken off, slipped at least 100ft (30m) away from the bow and is "moving significantly," pounded by 19ft (6m) swells.

"There has been a significant discharge of containers and container debris from the ship," said Maritime New Zealand spokesman Ross Henderson, warning that the storm will continue for another three to four days.

Oil clean up teams had "been activated to respond to the potential release of oil from the ship and to treat any affected wildlife," he said in a statement.

Salvage crews have removed more than 1,100 tons of oil from the stricken vessel. But about 385 tons remain on board - about the same amount that has already leaked into the sea.

The Greek-owned Rena ran aground on Astrolabe Reef 14 miles (22km) from Tauranga Harbour on New Zealand's North Island on October 5, spewing heavy fuel oil into the seas, fouling beaches and killing up to twenty thousand sea birds in what has been described as the country's worst maritime environmental disaster.