Monday's collision triggered huge fires aboard the two ships, which were still burning two days later although they had been greatly reduced by a huge high-seas fire-fighting effort.

Investigations are continuing into how the Portuguese-flagged Solong cargo ship ran into the US-flagged Stena Immaculate which was anchored about 13 miles (20 kilometres) off the coast of the northeastern UK port of Hull.

UK police on Tuesday arrested the captain of the Solong, owned by German company Ernst Russ, on suspicion of manslaughter after the crash left one crew member missing, presumed dead.

"Ernst Russ confirms that the master is a Russian national," the Germany-based company said in a statement to AFP. "The rest of the crew are a mix of Russian and Filipino nationals."

The detained 59-year-old "remains in our custody having been arrested on suspicion of gross negligence manslaughter", Humberside police said.

Prime Minister Keir Starmer paid tribute to the rescue services in parliament on Wednesday, hailing "the bravery and dedication of all those responding to the ship collision off the East Yorkshire coast."

- Deficient equipment -

Last year Irish officials had found the Solong's emergency steering compass was deficient during a routine safety check, documents from the port authorities revealed.

It was one of 10 deficiencies noted in the inspection carried out when the ship visited Dublin in July 2024.

Something went "terribly wrong" for the crash to happen, but that there was "no evidence" of foul play, UK under-secretary for transport Mike Kane told MPs on Tuesday.

At least one tank containing some of the 220,000 barrels of jet fuel on board the Stena Immaculate was "ruptured", according to the tanker's US-based operator Crowley.

That sparked concerns for the surrounding environment and wildlife.

But there were "no further reports of pollution to the sea" following the "initial incident", Virginia McVea, Chief Executive of the Maritime and Coastguard Agency said.

Environmental campaign group Greenpeace, which had raised the alarm at the potential fallout in the wildlife rich Humber estuary, said an "environmental disaster may have been narrowly averted".

"Though an incident of this scale is rare, any such accident has the potential to become an environmental disaster," said Paul Johnston from the Greenpeace Research Laboratories.

- Concerns for wildlife -

Ana Cowie, marine advocacy manager at Yorkshire Wildlife Trust, told AFP they were still waiting to see what the environmental impact would be, although the trust had not received any reports of damage as yet.

"Because the oil (jet fuel) is lighter than water, it will create a film on the top and there is a risk of wildlife like birds, like porpoise, like dolphins ingesting that," she said.

There were also concerns that any oil could coat the wildlife's feathers and fur which would mean they were not able to fly, or feed themselves properly.

Aerial AFP images on Tuesday showed a gaping gash in the middle of the 140-metre-long (460 foot) tanker, and smoke billowing from the Solong, which was reduced to an almost burnt-out wreck.

Ernst Russ said Wednesday that "the vessel continues to emit smoke with occasional reports of flames" and it was working with the investigators "to more fully understand these events".

Maritime expert Abdul Khalique told AFP such a collision between an anchored ship and another on a "routine" journey was "very rare".

"It's still not known why was MV Solong unable to take action to avoid collision," said Khalique, head of the Maritime Centre at Liverpool John Moores University.

The vessel missed "multiple opportunities" to change course, he said, adding the full reasons for the crash would only emerge after the investigation.

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

According to data collected by website VesselFinder, the impact of the crash was so severe that it displaced the tanker by "more than 400 metres".