Fires are still burning on an oil tanker carrying jet fuel and a cargo ship carrying highly toxic chemicals that collided in the North Sea on Monday morning.

One crew member is still unaccounted for and the search has been called off, HM Coastguard said.

The collision involved the US-registered Stena Immaculate – which was transporting fuel on behalf of the US military – and the Portuguese-flagged Solong.

Coastguard Divisional Commander Matthew Atkinson said 36 people had been rescued, with one person taken to hospital. The missing crew member had been onboard the Solong, he said.

One of the men who was on the oil tanker told BBC News that the Solong came out of the blue and collided with the Stena Immaculate at 16 knots.

The crew scrambled to the life rafts taking only what they had on them.

Efforts to assess any environmental damage are still ongoing after it was confirmed some jet fuel had leaked into the sea off the Humber Estuary.

Maritime firm Crowley, which manages the Stena Immaculate, said there had been “multiple explosions onboard” when the vessel suffered a ruptured cargo tank.

A US official confirmed the tanker had been carrying jet fuel “in support of the Department of Defense” but said the incident would not impact operations or combat readiness.

A Coastguard rescue helicopter was sent to the scene, as well as four lifeboats and nearby vessels with fire-fighting capability.

Svitzer managing director Michael Paterson said the marine services group had sent four boats which had been “firefighting since they arrived and dealing with the flames”.

“The oil and jet fuel presents a problem for them. It’s obviously flammable, but there’s also fumes and toxins,” he said.

“The priority for us was to save the crew. I’ve seen some serious incidents, but this is one of the most serious.”

The Marine Accident Investigation Branch said a team of inspectors and support staff had started gathering evidence and was undertaking a preliminary assessment to determine its next steps.

Divisional Commander Atkinson said: “An assessment of any required counter pollution response is being carried out by the Maritime and Coastguard Agency.

“The Secretary of State’s Representative for Maritime Salvage and Intervention is working with the salvors and insurance companies.”

According to MarineTraffic, the Stena Immaculate had travelled from the Greek port of Agioi Theodoroi and was anchored by the Humber Estuary.

The Solong had been sailing from the Scottish port of Grangemouth to Rotterdam, in the Netherlands.

The cargo ship had been carrying 15 containers of the chemical sodium cyanide when it collided with the tanker.

Sodium cyanide has a variety of commercial uses, from being used to plate metals to dye production. It is highly soluble in water and can be toxic as it affects the uptake of oxygen.

It is not clear whether any sodium cyanide from the cargo ship has leaked into the sea.

But if it has come into contact with water, “there is a risk of some hydrogen cyanide gas being produced which could present a risk to any of those involved in rescue”, according to Alastair Hay, professor of environmental toxicology at Leeds University.

The Stena Immaculate had been carrying jet fuel when it was hit, said Crowley.

Jet fuel has a high boiling point, so it will evaporate slowly. It is also relatively toxic, so marine life that comes into contact with it may be killed.

Greenpeace said it was “extremely concerned” about “multiple toxic hazards” from the North Sea ship collision.

Downing Street said details of the cause of the collision were “still becoming clear”.

The prime minister’s official spokesman said it was an “extremely concerning situation”.

He said: “We thank the emergency services for their rapid response. I understand the Department for Transport is working closely with the coastguard to help support the response to the incident.”

P&O Ferries said the Humber Estuary remained closed to all traffic and departure times from Hull and Rotterdam remained unconfirmed on Monday evening.

Additional reporting by Jonathan Josephs and Holly Phillips

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