Feb 23 (Reuters) - Trinidad and Tobago's government on Friday confirmed the refined product leaking from a barge that struck a reef and overturned off the coast of Tobago was tested and determined to be a type of fuel oil.

The leak, which remains unplugged and is spreading to the Caribbean Sea threatening several countries' coasts, was first spotted by Trinidad's Coast Guard on Feb. 7 after a barge towed by a tugboat capsized near Tobago's shore.

"Analyses of the hydrocarbon discharge collected in Tobago indicates that the samples are characteristic of a refined oil," Trinidad's energy ministry said in a release, characterizing it as "intermediate fuel oil."

Intermediate fuel oil can be used as a bunker fuel to power combustion engines.

The pace at which the fuel is flowing from the barge has slowed considerably, Allan Stewart, the head of Tobago's emergency department, told Reuters.

The fuel has been leaking for more than three weeks and has stained Tobago's coastline, affecting fishing and tourism, and has entered the waters off Grenada.

"We are working hard to ensure the hydrocarbons do not get to the more sensitive southwest part of the island, where there are the popular tourist beaches," Stewart added.

Containment booms are so far holding the spill. Tobago has been using skimmers and other equipment to mop-up the fuel, according to Stewart.

The barge carried as much as 35,000 barrels of fuel oil, Tobago officials have said.

The ship sailed from Panama and it was bound for Guyana, officials have said. But monitoring service TankerTrackers.com and investigative news outlet Bellingcat said after reviewing satellite photos that the vessels were near Venezuela's shore days before the spill was first reported in Tobago.

"Imagery discovered by Bellingcat shows that the barge began leaking oil as early as February 3, immediately after leaving (Venezuela's) Pozuelos Bay, and that it appears to have capsized by the morning of February 6," it said this week.

Venezuela has denied that the barge originated from the Bolivarian Republic. (Reporting by Curtis Williams in Houston)