SYDNEY, Australia — An oil spill from a cargo ship that ran aground near a World Heritage site in the South Pacific is spreading, alarming environmentalists and government officials about the threat to the delicate local ecosystem and to people living there.

The Hong Kong-flagged ship, Solomon Trader, was carrying more than 770 tons of heavy fuel oil when it ran aground last month on Rennell Island, one of the Solomon Islands, which Unesco says is the largest raised coral atoll in the world. The ship is leaking just outside the boundaries of the World Heritage site, called East Rennell.

Officials in Australia, which has a close relationship with the Solomon Islands, said this week that oil had spread along more than three miles of coastline and into the surrounding sea, and that it was approaching the boundaries of the heritage site. The source of the leak has yet to be found.

Australia initially sent experts to help monitor the spill, but Foreign Minister Marise Payne said on Sunday that the country would step up efforts and send equipment, vessels and experts to help contain the damage to Rennell Island, which is home to animal species found nowhere else.