Media playback is unsupported on your device Media caption Ross Henderson, Maritime New Zealand: "We don't yet know what amount of oil may have been released"

A cargo ship which ran aground off the coast of New Zealand three months ago has broken in two, spilling containers and threatening a new oil spill.

Heavy seas snapped off the stern section of the Greek-owned Rena, which leaked large amounts of fuel on becoming stranded in October.

Up to 300 containers have been washed overboard, with most expected to sink.

While a new oil leak is feared in coming days, clean-up teams expect it to be smaller than the initial escape.

The stranding has been described as New Zealand's worst maritime environmental disaster.

The Rena struck the well-marked Astrolabe Reef off the North Island resort area of Tauranga on 5 October.

Its captain and other senior officers face up to 16 charges relating to the wreck.

'Significant release'

On Saturday night, the ship broke in two after being hit by waves of up to 6m (20ft), heavy winds and heavy seas, Maritime New Zealand (MNZ) spokesman Ross Henderson told the BBC.

We are gearing up for an expected release of oil and container debris on shore Ross Henderson, Maritime New Zealand

"The forward section remains firmly grounded on the reef, but the rear section has broken away," he said, adding that the two parts of the ship were now 20-30m apart.

Mr Henderson spoke of a "significant release of containers and container debris".

Tonnes of milk powder from one of the containers have spilled into the sea, making the water around the wrecked vessel murky, the New Zealand Herald reports.

Timber has also been sighted among the debris.

Container recovery company Braemar Howells said it believed 200-300 containers out of 800 still aboard the ship had washed overboard when the ship split.

"Of those 20% will float - the remainder will sink," said Braemar Howells spokeswoman Claudine Sharpe.

It is likely the stern section will capsize and sink, which could make recovering any further containers considerably more difficult, said MNZ salvage adviser Jon Walker.

Oil threat

Hundreds of tonnes of fuel have spilled into the sea since the ship ran aground, killing hundreds of seabirds.

Salvage crews have removed more than 1,100 tonnes of oil from the stricken vessel, but 385 tonnes remain aboard.

Environment Minister Nick Smith said the efforts of rescue 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," he told reporters in Tauranga.

Alex van Wijngaarden, New Zealand's national on scene commander, said: "Any oil coming ashore in the coming days is expected to be much less the amount that washed up after the Rena first went aground."

Quoted by the Herald, he said people should "exercise their common sense and not swim or surf where there is likely to be containers, debris or oil coming ashore".