An official from the European Union’s naval mission Aspides has said that vessels have been receiving VHF transmission from Iran’s Revolutionary Guards saying “no ship is allowed to pass the Strait of Hormuz”, Reuters reports.

The strait is the world’s most vital oil export route, which connects the biggest Gulf oil producers, such as Saudi Arabia, Iran, Iraq and the United Arab Emirates, with the Gulf of Oman and the Arabian Sea.

The official, who spoke to Reuters on condition of anonymity, said Iran had not formally confirmed any such order. Tehran has for years threatened to block the narrow waterway in retaliation for any attack on the Islamic Republic.