A cargo ship which ran aground off the coast of New Zealand three months ago has broken in two, spilling containers and other debris into the sea. Officials said heavy seas had snapped off the stern section of the Greek-owned Rena, which leaked large amounts of oil on becoming stranded in October. Clean-up teams have been alerted amid fears the ship could leak some of the remaining oil on board. The stranding has been described as New Zealand's worst maritime disaster. The Rena struck the well-marked Astrolabe Reef off the North Island resort area of Tauranga on 5 October. 'Fragment of the risk' "Overnight, the vessel was hit by wave action of up to 6m, heavy winds and heavy seas. That's caused the ship to break into two parts," Maritime New Zealand (MNZ) spokesman Ross Henderson told the BBC. "The forward section remains firmly grounded on the reef, but the rear section has broken away." Mr Henderson said the two parts of the ship were now 20-30m apart. "What we do know is there's been a significant release of containers and container debris. "We don't yet know what amount of oil may have been released, but we are gearing up for an expected release of oil and container debris on shore, and have response teams ready to go once that starts happening." Container recovery company Braemar Howells said it believed 200-300 containers of the some 800 which had remained on the ship went overboard when the ship split. Meanwhile, MNZ salvage advisor Jon Walker said it was likely that the stern section would capsize and sink, which could make recovering any further containers considerably more difficult. Speaking at a news conference in Tauranga, Environment Minister Nick Smith said the efforts of MNZ officials over the past three months had limited the damage caused by the break-up. "The risk to the environment is a fragment of what it was, with at the most tens of tonnes of oil rather than hundreds of tonnes that potentially could be spilled." Salvage crews have removed more than 1,100 tons of oil from the stricken vessel, but 385 tonnes remain onboard. Hundreds of tonnes of fuel have already spilled into the sea, killing more than 20,000 seabirds in the area. The captain and other senior officers face up to 16 charges relating to the wreck.