UNITED NATIONS, July 10 (Reuters) - The United Nations and the European Union sent experts on Thursday to the Philippines to assess the capsize of a ferry last month with large quantities of a highly toxic chemical on board, a U.N. spokeswoman said.

The joint team will spend a week determining the effects of the disaster of June 21, when the ferry ran aground and capsized off Sibuyan Island during a typhoon, said the spokeswoman, Michele Montas.

Only 56 people were believed to have survived, and at least 700 people were believed trapped inside the vessel, which was carrying 10 tonnes of endosulfan, a toxic pesticide.

"If not handled properly, this could be a disaster upon a disaster," Vladimir Sakharov, chief of a U.N. environmental emergency response unit, said in a statement released by the U.N. News Service.

"Leakage of the ferry's toxic cargo would cause major ecological damage and thereby have a terrible impact on the livelihoods of people living in the region," he said.

The joint team, comprised of a marine chemist, an eco-toxicologist and a civil protection expert, went at the request of Philippine authorities, Montas said.

They will review the situation and help determine what further measures should be taken, she said.

Philippine authorities have said they plan to refloat the ferry. Efforts to remove the trapped bodies came to a halt when shipping officials said the cargo included the toxic pesticide.

Water samples taken from the sea nearby have shown no contamination so far, but a fishing ban has been put into effect.

(Reporting by Ellen Wulfhorst; Editing by Sandra Maler)

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