UK royal marines have boarded a VLCC off Gibraltar after suspecting it was en route to Syria in breach of EU sanctions.

The government of Gibraltar said authorities had reason to believe the 301,000-dwt Grace 1 (built 1997) was carrying crude to the Banias refinery, the BBC reported.

Some reports said it had a cargo of Iranian oil, but this was unconfirmed.

Later on Thursday, the government of Gibraltar told TradeWinds: "A police investigation is ongoing. The ship is still at anchor and the crew are still on board but are being interviewed at this stage by Royal Gibraltar Police and Her Majesty’s Customs officers."

There are no longer any marines on board.

The government added that the 28 crew are mainly Indian nationals, though there some Ukrainian and Pakistani nationals too.

"In total about 60 officers worked together on last night’s operation," it said.

"The Royal Gibraltar Police and HM Customs currently have approximately 50 officers on board conducting various roles."

Police have yet to establish the ownership of the vessel with "certainty."

"This will no doubt unfold during the course of the investigation," the government said.

The vessel is listed as managed by Iships Management of Singapore, which has been contacted for comment.

Shipping databases list the owner as Russian Titan Shipping Lines of the Ukraine, with its registered address a PO Box in Dubai. The Grace 1 appears to be the only vessel in its fleet.

The Gibraltar government published regulations on Wednesday to enforce the sanctions against the tanker and its cargo.

Refinitiv Eikon mapping indicates the ship sailed from Iran, Reuters said.

If the cargo is confirmed as Iranian crude, attempted delivery to Syria could also be a violation of US sanctions on Iranian oil exports.

Mapping data shows the tanker went the longer route around South Africa instead of via Egypt's Suez Canal.

The refinery is subject to European Union sanctions against Syria, according to Fabian Picardo, chief minister of the Gibraltar government.

"Be assured that Gibraltar remains safe, secure and committed to the international, rules-based, legal order," he added.

The ship was held on Thursday morning by the marines, together with Gibraltar port and law enforcement agencies.

It was boarded while stopped outside the port limits to take on food and other supplies, the Gibraltar Chronicle reported.

Picardo said he had written to the presidents of the European Commission and European Council to give details of the sanctions that have been enforced.

The UK Foreign Office said in a statement: “We welcome this firm action by the Gibraltarian authorities, acting to enforce the EU Syria Sanctions regime.”

The ministry of defence said a little under 30 marines were involved.

Their boarding skills were used to take control of the vessel at sea, it added.