Maritime authorities in New Zealand say the container ship that has been grounded on a reef for nearly a month could soon break apart.

Salvage crews say a large crack in the hull of the Liberian-flagged container ship Rena worsened after another night of bad weather.

The Rena has leaked more than 300 tons of fuel oil from its tanks since it ran aground off New Zealand's north coast on October 5. The oil has spread to nearby beaches, killing thousands of birds. Salvage crews have removed about 1,200 tons of oil from the crippled ship, but at least 350 tons remain in its tanks.

If the Rena breaks apart, a barge will tow the vessel to shallow waters.

The captain of the Rena and his navigation officer, both Philippine nationals, appeared in a courtroom Wednesday where they were charged with discharge of a harmful substance, which carries a maximum penalty of two years in prison.

The pair already faced charges of operating a vessel in a way that caused unnecessary risk, which carries a maximum penalty of 12 months in prison.