UK police on Friday again extended the detention of the captain of a cargo ship that struck a tanker in the North Sea, citing the "complexities" of the case.

The Russian captain was arrested on Monday on suspicion of gross negligence manslaughter after his ship, the Solong, slammed into the tanker anchored off the coast of Hull in northeastern England, setting both ships ablaze and leaving one sailor presumed dead.

Police were granted two extensions on Wednesday and Thursday to allow more time to question the 59-year-old captain due to "the complexities of the incident," the local Humberside police force said in a statement.

Police cited the location of both vessels at sea as one of the complications facing the probe, with the ships on fire for several days after the incident, requiring a massive firefighting response.

While all crew onboard the jet fuel-laden tanker, the US-flagged Stena Immaculate, were safely rescued, one sailor from the Portuguese-flagged Solong remains missing and presumed dead.

Although the government has ruled out foul play, investigators are still determining the causes of the crash, in which the Solong never deviated from its course and slammed into the Stena at 16 knots an hour.

Pockets of fire were still being reported on the deck of the Solong on Thursday evening, according to the UK Coastguard.

"Extensive lines of enquiry are continuing," police said.

Salvage teams boarded the vessels on Thursday to carry out initial damage assessments. (AFP)