There were also growing fears that any spill from the collision could harm the local environment and coastline, home to seals, porpoises and some protected waders and waterfowl.

Images on the BBC Tuesday showed a large hole in the side of the Stena Immaculate tanker as huge plumes of thick, black smoke rose from the stationary ships with smaller boats dousing the vessels with water.

The fires were "still going on" nearly 24 hours after the Portuguese-flagged Solong cargo ship ploughed into the Stena Immaculate tanker, anchored about 10 miles (16 kilometres) off the northeastern port of Hull, nearby Grimsby port chief executive Martyn Boyers told AFP.

One crew member was also still missing, he said.

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

Crowley, the US-based operator of the tanker, said the crash had "ruptured" the ship's tank "containing A1-jet fuel" and triggered a fire, with fuel "reported released".

The UK government's Marine Accident Investigation Branch has launched a probe into Monday's accident to determine the next steps.

The UK Coastguard halted search operations late on Monday after rescuing 36 crew members from both ships. It was not immediately clear if the search had resumed early on Tuesday.

"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 the Coastguard.

The Stena Immaculate was carrying around 220,000 barrels of jet fuel, according to the maritime information service, Lloyd's List Intelligence.

The Solong was laden with 15 containers of sodium cyanide, it added, but authorities have not confirmed that and it is not known if any of the flammable compound had leaked.

A spokesperson for Prime Minister Keir Starmer, who was hosting his weekly cabinet meeting on Tuesday, called the situation "extremely concerning".

Quoting government sources, the Daily Telegraph said there was nothing so far to indicated that "foul play" had caused the crash, but it could not be ruled out.

- 'Toxic hazards' -

A spokesman for the government's Marine Accident Investigation Branch said a team sent to Grimsby was "gathering evidence and undertaking a preliminary assessment".

Dutch maritime servicing company Boskalis told the ANP news agency it had been tasked with salvaging the Stena Immaculate and was "fully mobilising".

Four ships with firefighting capacity were on their way to the site, a Boskalis spokesperson said, adding the tanker would need to be "cooled down" before the fire could be extinguished.

The investigation was being led by US and Portuguese authorities, as the ships were flagged from their countries, housing minister Matthew Pennycook told Times Radio.

"We're obviously very alive to the potential impact on the environment," he said, but added the Coastguard was well equipped to deal with any oil spills.

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

"Most of it will evaporate quite quickly and what doesn't evaporate will be degraded by microorganisms quite quickly," he added.

Paul Johnston, a senior scientist at the Greenpeace Research Laboratories at Exeter University, said: "We are extremely concerned about the multiple toxic hazards."

Sodium cyanide is "a highly toxic chemical that could cause serious harm", he explained.

- 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.

"It's more the nature, birds... if it gets down to the nature reserve," one local woman, Laura Scrimshaw, 47, told AFP in Grimsby, which is located on the south bank of the Humber estuary, home to several nature reserves.

"We don't want to see wildlife dying. It's a chain of events, it affects the wildlife which could then affect other" species, she added.

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