The fire department in the German port city of Hamburg issued a warning on Sunday morning as a large fire billowed out smoke and hazardous gases.

Two warehouses at a truck stop in the Rothenburgsort district of the city had been in flames since the early hours of the morning, according to German media.

The fire and smoke were classified as an "extreme danger" in an official warning.

People evacuated as toxic gasses leak

Local public broadcaster NDR reported that hydrogen sulfide was leaking from the site, according to initial investigations. Firefighters were also reportedly only able to be on the scene with respirators.

Firefighters were still tackling the blaze several hours after it was first reported.

"Hamburg's city center is completely blacked out," a fire department spokesperson told German news agency DPA, following earlier reports that the plumes of smoke were heading for the center.

A police spokesperson told Reuters that it was not yet possible to fully assess the danger posed by the fire, but added that 140 people had been evacuated from the area.

Part of the problem was the dark sky which made it difficult to determine what was smoke and what was just cloud.

Blaze causes disruption to trains

Hamburg residents were told to keep their doors and windows closed and were told to turn off air conditioning systems and close vents.

German rail operator Deutsche Bahn said that it had closed the route between Hamburg and the nearby town of Büchen due to the fire.

Trains between Hamburg and Berlin were also being diverted and could face delays of up to 60 minutes.

NDR reported that there were numerous explosions and that six fire trucks had been deployed to fight the blaze.

Emergency services were not able to give information on what exactly was being burned inside the warehouses or on what caused the fire.