Two large oil tankers were on fire on Friday after colliding in waters near Singapore, the world’s biggest refuelling port, with two crew members airlifted to hospital and others rescued from life rafts, authorities and one of the companies said.

The Singapore-flagged tanker Hafnia Nile and the Sao Tome and Principe-flagged tanker Ceres I were about 55 km (34 miles) northeast of the Singaporean island of Pedra Branca on the eastern approach to the Singapore Straits, the Maritime and Port Authority of Singapore (MPA) said.

The owner of the Hafnia Nile said the vessel was involved in a collision with the Ceres I. The 22 crew of the Hafnia Nile and the 40 on the Ceres I were all accounted for, said the MPA, which was alerted to the fire at 6:15 a.m. (2215 GMT)

Photographs released by the Singapore Navy showed thick black smoke billowing from one tanker and crew being rescued from life rafts and flown to hospital.

The environmental authorities in neighbouring Malaysia said they had been informed to prepare for further action in case of oil spills.

The 74,000 deadweight-tons capacity Panamax tanker Hafnia Nile (IMO 9766217) was carrying about 300,000 barrels of naphtha, according to ship-tracking data from Kpler and LSEG.

It was not immediately clear what fuel Ceres I (IMO 9229439) was carrying. The tanker is a very-large-crude-carrier (VLCC) of 300,000 deadweight-tons capacity and was last marked as carrying Iranian crude between March to April, ship-tracking data showed.

Before the fire, the Ceres I had been at the same location since July 11, according to LSEG shipping data.

The area is known to be used by so-called dark fleet ships for the transfer of Iranian oil in contravention of U.S. sanctions, said Michelle Wiese Bockmann, principal analyst at Lloyd’s List Intelligence.

“The Ceres I has repeatedly been involved in transferring or shipping Iranian oil in breach of U.S. sanctions,” she said.

Shipping sources have said the tanker was also involved in transporting Venezuelan oil to China in recent years.

The China-based owner of the Ceres I could not immediately be reached for comment. China has repeatedly said it opposes unilateral sanctions.

Singapore is Asia’s biggest oil trading hub and the world’s largest bunkering port. Its surrounding waters are vital trade waterways between Asia and Europe and the Middle East.