Houthi fighters in Yemen have claimed responsibility for the attack. Britain’s ambassador to the country warned it risked an “environmental catastrophe”.
A tanker carrying 150,000 tonnes of oil has been damaged in the biggest attack on Red Sea shipping in weeks.
The Greek-flagged MV Sounion was reportedly hit by at least four projectiles as men in several small boats opened fire on the vessel about 90 miles off the coast of the Yemeni port city of Hodeida.
Officials said drones or missiles may have been used in the assault.
The attack on Wednesday caused a fire on board the tanker, which also lost power.
Its crew of 25 men were forced to abandon ship and were rescued by a French military vessel.
The MV Sounion then floated adrift, although officials confirmed on Thursday it had since been anchored.
A spokesperson for the EU’s Aspides military operation in the Red Sea warned on Thursday that the tanker posed an “environmental hazard” due to the amount of oil on board.
They added that its crews had also destroyed an unmanned drone boat in the area.
Houthi officials in Yemen confirmed its forces targeted the Sounion, along with Panama-flagged ship the SW North Wind I, which suffered minor damage in a separate attack.
The group, which controls large areas of the country, has been targeting shipping in the region since last autumn in solidarity with the Palestinian people and to put pressure on Israel to end its assault in Gaza.
Houthi leader Abdul Malik al-Houthi said on Thursday: “From this week’s operations targeting ships violating blockade on vessels heading to Israel, a ship was adrift… after it malfunctioned because of strikes”.
The Greek shipping ministry said the vessel had been sailing from Iraq to Agioi Theodoroi in Greece with a crew of two Russians and 23 Filipinos.
Four private security personnel were also said to be on board.
Greek Shipping Minister Christos Stylianidis described the attack as “a flagrant violation of international law and a serious threat to the safety of international shipping”.
Britain’s ambassador to Yemen Abda Sharif said: “Another Houthi attack threatens Yemen’s coastline, fishing industry and environmental catastrophe.”
Delta Tankers, which owns the Sounion, said it was working on plans to move the ship to a safer destination for further checks and repairs.