Emergency workers in Trinidad and Tobago are racing to clean up a massive oil spill after a mystery vessel ran aground near the Caribbean islands, casting a pall over Carnival tourism.

The spill was "not under control" as of Sunday, said Prime Minister Keith Rowley, who added that the country is grappling with a national emergency.

The mystery vessel capsized Wednesday, having made no emergency calls, with no sign of crew, and no clear sign of ownership.

Rowley on Sunday declared a national emergency as oil leaking from the vessel affected nearly 10 miles of coastline.

"Cleaning and restoration can only begin as soon as we have the situation under control. Right now the situation is not under control," the prime minister told journalists.

Divers have so far been unable to plug the leak.

Hundreds of volunteers have been toiling since Thursday to halt the spread of the oil, and the government has asked for even more to lend a hand. Images and video released by the government showed crews working late into the night Sunday.

The leak has damaged a reef and Atlantic beaches, and residents of the village of Lambeau have been advised to wear masks or temporarily relocate.

The government posted satellite imagery on social media, showing affected areas.

"The satellite imagery reveals a distinctive silver-like slick emanating from the overturned wrecked vessel. Additionally, there are noticeable streaks of a thick, black-like substance accompanying the spill," the post says.

The spill comes at the height of Carnival, threatening the tourist business that is crucial to the dual-island nation's economy.

Just how badly tourism will be affected remains unclear. A cruise ship carrying 3,000 people docked in Tobago on Sunday.

Rowley said the mystery vessel might have been involved in "illicit" business, adding: "We don't know who it belongs to. We have no idea where it came from, and we also don't know all that it contains."

Divers spotted the name "Gulfstream" on the craft's side and have identified a length of cable, possibly indicating it was in the process of being towed, Rowley said.

The island's Emergency Management Agency said there were no signs of life on the vessel, which is around 330 feet in length. The agency has posted dozens of images and videos on social media showing the ship and crews scrambling to contain and clean up the oil spill.

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• Oil Spill
• Caribbean