HM Coastguard rescued 36 crew members from the Stena Immaculate tanker and Solong container vessel, with one taken to hospital.

"One crew member of the Solong remains unaccounted for, after an extensive search for the missing crew member sadly they have not been found and the search has ended," said Matthew Atkinson, Divisional Commander for HM Coastguard.

Images showed a huge plume of thick, black smoke and flames rising from the scene about 10 miles (16 kilometres) off the east England coast, sparking concerns of "multiple toxic hazards".

The Stena Immaculate tanker was "anchored off the North Sea coast near Hull... (and) was struck by the container ship Solong", the Stena's US-based operators Crowley said in a statement.

The Stena was on a short-term US military charter with Military Sealift Command, according to Jillian Morris, the spokesperson for the command that operates civilian-crewed ships providing ocean transport for the US Defense Department.

Crowley said the impact of the collision "ruptured" the tanker "containing A1-jet fuel" and triggered a fire, with fuel "reported released".

It was carrying around 220,000 barrels of jet fuel while the Solong was carrying 15 containers of sodium cyanide, according to the Lloyd's List information service, but it is not known if any of the flammable compound had leaked.

The two vessels were still on fire 12 hours after the collision, said the coastguard.

A spokesperson for UK Prime Minister Keir Starmer called the situation "extremely concerning".

All crew members aboard the Stena Immaculate were confirmed to be alive, a spokeswoman for the tanker's Swedish owner, Stena Bulk, told AFP.

- 'Toxic hazards' -

A spokesman for the government's Marine Accident Investigation Branch spokesperson said "Our team of inspectors and support staff are gathering evidence and undertaking a preliminary assessment of the accident to determine our next steps".

Ivor Vince, founder of ASK Consultants, an environmental risk advisory group, told AFP that "the good news is it's not persistent, it's not like a crude oil spill".

"Most of it will evaporate quite quickly and what doesn't evaporate will be degraded by microorganisms quite quickly", he added, though warning that "it will kill fish and other creatures".

Paul Johnston, a senior scientist at the Greenpeace Research Laboratories at Exeter University, said "we are extremely concerned about the multiple toxic hazards these chemicals could pose to marine life."

The jet fuel entered the water close to a breeding ground for harbour porpoises, sodium cyanide is "a highly toxic chemical that could cause serious harm", he added.

- Humber traffic suspended -

All vessel movements were "suspended" in the Humber estuary that flows into the North Sea, according to Associated British Ports (ABP), which operates in the Ports of Hull and Immingham in the region.

The German Central Command for Maritime Emergencies said it was also dispatching a vessel capable of fire fighting and oil recovery.

The alarm about the crash near the port city of Hull in East Yorkshire was raised at 0948 GMT.

A coastguard helicopter, a plane, lifeboats from four towns and other nearby vessels were part of the large rescue operation, UK Coastguard said.

Grimsby native Paul Lancaster, a former seaman, told AFP that "I don't understand how two ships that big could collide".

"There must have been a massive engineering problem," he said outside a pub in Grimsby.

- Collisions rare -

Collisions remain rare in the busy North Sea.

In October 2023, two cargo ships, the Verity and the Polesie, collided near Germany's Heligoland islands in the North Sea.

Three people were killed and two others are still missing and considered dead.

In October 2015, the Flinterstar freighter, carrying 125 tonnes of diesel and 427 tonnes of fuel oil, sank after colliding with the Al Oraiq tanker eight kilometres (five miles) off the Belgian coast.