A tanker caught fire overnight in Romanian waters near the port of Sulina, probably after a drone strike, said Adrian Mihalcea, head of the Free Seafarers' Union.
It is not yet clear whether it was an attack or an accident.
Mihalcea, who also represents the International Transport Workers' Federation in Romania, said the evidence so far points more to an attack than an accident. This has not been fully confirmed.
If a drone strike is confirmed, it will be the first of its kind in the territorial waters of a NATO country.
The ship, YARA J, sails under the flag of the Comoros. Unconfirmed reports say the fire broke out after an explosion. The crew, at first said to be only Syrians, has been evacuated. The tanker is still moving under its own power towards the port of Constanta.
Ship-tracking data show YARA J was carrying chemical and oil products. Unconfirmed reports say they were bound for Ukraine. The ship is believed to have been sailing from Turkey to the Ukrainian port of Izmail, where it was due to load cargo.
YARA J has IMO number 9221803. It is 118 metres long and 19 metres wide. Its last recorded port call was Sulina, Romania, on 26 September.
Sulina is known as the easternmost port of the EU. The border with Ukraine runs through the middle of Lake Sulina. Under maritime law, territorial waters extend 12 miles from the coast.
On 6 October, drones hit Alfa Watan and Able in Bulgaria's exclusive economic zone. Burgas maritime expert Doychin Doychev, whom we quoted on 9 October, said the sunken Alfa Watan was sailing to Sulina with about 3,000 tonnes of salt.
As a NATO member, Romania can ask to invoke Article 5 of the alliance's treaty. It says an attack on one member is an attack on the whole alliance.