Marine crews were preparing Sunday for an operation to extract oil from a container ship that is stranded on a reef near New Zealand.

The 775-foot (236-metre) Liberia-flagged "Rena" struck the Astrolabe Reef about 12 nautical miles from Tauranga Harbour early Wednesday, and has been foundering there since. The ship has been leaking fuel, leading to fears it could cause an environmental disaster if it breaks up further.

Heavy swells and gale-force winds are forecast for the area from Monday.

The ship has about 1,700 metric tons of fuel on board, according to Maritime New Zealand, the agency responsible for shipping in the region. The agency believes that so far, up to 30 tons have leaked into the Bay of Plenty, a spot noted for its fishing, diving and surfing.

The agency said in an update late Sunday that a barge, the Awanuia, had pulled up alongside the Rena and was to begin pumping fuel from the stricken ship within a few hours. The operation is expected to last at least two days, although it could be delayed by bad weather.

"The weather is expected to deteriorate in the coming days, so we are working around the clock to remove the oil," the agency said.

Salvage experts and naval architects are on board to monitor the ship, the agency said, and sensors should indicate if the ship is in danger of breaking apart.

"The top priority is to first remove the oil, then lighten the vessel by removing the containers, and finally, move the ship off the reef," the agency said.

A Navy vessel, the Endeavour, will be used as a command platform for the operation. About 200 people are working on the response, and New Zealand's defense force has about 300 people standing by in case beach cleanups are needed.

So far, eight sea birds _ six little blue penguins and two shags _ have been rescued from an oil slick that extends about three miles from the boat.

On Sunday, New Zealand Prime Minister John Key visited the area and demanded answers.

"This is a ship that's plowed into a well documented reef in calm waters in the middle of the night at 17 knots. So, somebody needs to tell us why that's happened," he told reporters.

In a statement, the owners of the ship, Greece-based Costamare Inc., said they are "cooperating fully with local authorities" and are making every effort to "control and minimize the environmental consequences of this incident."

The company did not offer any explanation for the grounding.

"Evaluations so far indicate that hull stresses are within allowable limits and that there is no deterioration of the ship's condition," the company said, adding that minimizing damage to the New Zealand coastline was its priority.

Environmental agency Greenpeace denounced the spill and what it claims is a slow response.

"This is an unfortunate illustration of just how difficult it is to deal with oil spills at sea," said Greenpeace spokesman Steve Abel. "Even a slow, and relatively accessible oil spill like this one has clearly stretched New Zealand's response capability to its limits."

"It is also a potential disaster for the blue whales and dolphins presently calving in the area, as well as numerous other marine species."

The Rena was built in 1990, according to the owners, and was carrying 1,351 containers when it ran aground.

As well as oil, authorities are also concerned about some potentially dangerous goods aboard, including four containers of ferrosilicon. Authorities say they will make it a priority to remove those goods as part of their recovery operation.