Image copyright AFP Image caption The US Navy says it is working out how to extract the ship

A US Navy minesweeper is stuck on a coral reef off the Philippines after running aground early on Thursday.

The USS Guardian struck the reef in the Sulu Sea south-east of Palawan island after completing a port call at the former US naval base of Subic Bay.

The reef is in the Tubbataha National Marine Park, designated a World Heritage Site by Unesco.

Efforts were made to free the Guardian at high tide on Friday but were not successful, said the US Navy.

In a statement, it said the ship had 79 crew on board when it ran aground, but that 72 were being transferred to a smaller vessel "as a precautionary measure".

"A small complement of engineering and bridge personnel will remain aboard and work with a US Navy team in an attempt to free Guardian with minimal environmental impact," it said.

The remaining crew, which includes the commanding officer, would also be transferred "if conditions become unsafe".

The cause of the grounding is still under investigation. No-one was hurt in the incident.

Philippine coast guards were being sent to check for damage to the reef, local media said. Park rangers had earlier been prevented from inspecting the ship, the reports said.

Washington and Manila are allies, and have been strengthening ties in recent months amid concern over territorial disputes with Beijing over the South China Sea.

According to Unesco, the area the ship is grounded is a "unique example of an atoll reef with a very high density of marine species". Part of it serves as a nesting site for birds and marine turtles, it says.

Environmental group Greenpeace was fined almost $7,000 (£4,380) in November 2005 for damaging a coral reef in the park after its flagship Rainbow Warrior II ran aground.

Greenpeace paid the fine but blamed the accident on outdated maps provided by the Philippine government.