"The tugboat sank Tuesday at dawn," Dan Panait, a port official, told reporters.

"We have already taken steps to reduce the spread of the oil using neutralising chemicals," Clara Adam, a local environment officer told the Mediafax news agency.

The tugboat, which was carrying wheat to the Romanian Black Sea port of Agigea, caught fire Monday morning, most likely following a short-circuit, shortly after leaving Ruse in the north of Bulgaria, a port authority official said.

All eight members of the Slovak crew "are safe and sound and took part Monday in efforts to put out the fire," the official said.