US Navy ship aground on Philippine Unesco coral reef 18 January 2013 Last updated at 05:00 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. The crew were "currently working to determine the best method of safely extracting the ship," the US Navy said. The cause of the grounding was under investigation, it said in a statement dated Thursday. 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, local media 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 Greenpeace paid the fine but blamed the accident on outdated maps provided by the Philippine government.