Two trains have collided head-on on a level crossing north west of Copenhagen, leaving five people critically hurt and 13 others with less serious injuries, medical officials say.

Police said 37 people were on board as the trains travelled early on Thursday on a line that connects the towns of Hillerød and Kagerup in the North Zealand area of north-east Denmark.

The Greater Copenhagen fire brigade’s Tim Ole Simonsen could not explain what caused the accident, but Danish TV reports that all the injured were brought to hospital by air or by ambulance.

Local mayor Trine Egetved said she was very disturbed by the accident.

Public broadcaster DR broadcast images of two trains, one yellow and another grey, both of which exhibited some damage to their front. 

On Thursday, the trains crashed at 06:29 local time (05:29 BST), and fire and rescue services were notified straight away, authorities said.

“Those who were injured so severely that they could not be moved to a safe place had been airlifted to the National Hospital in Copenhagen, ” Egetved, who is mayor of the Gribskov municipality, said.

She posted on Facebook about the local track being used by many Gribskov residents, workers and students.

She told public broadcaster DR that it was shocking that two trains could collide head-on “and we must ensure it never happens again”.

Prime Minister Mette Frederiksen, in a statement to the government-owned TV2 station, said she was “very concerned by the terrible train accident”.

The Accident Investigation Board from Denmark reached the scene in the morning, and Inspector Morten Pedersen of North Zealand police said they would be on site with the police to investigate.

It’s too early to determine the cause of the crash,” Pedersen told reporters.

The board’s unit manager for railways, Klaus Jensen, told broadcaster TV2 that investigators were looking “at all hypotheses,” which include “a failure in the signalling system, or whether there may have been a failure due to human factors”.

Claus Pedersson, safety director at Lokaltog – which operates the train line – told public broadcaster DR that several members of staff have been injured.

It is “one of the worst we can imagine in the railway industry”, and “right now we are gathering data” to understand what led to the accident, he said.

“We see accidents like this from time to time, and the most important thing is that we learn from them,” he added.

In another interview with TV2, Pedersson said early indications point to one of the train drivers braking the train and assisting passengers to evacuate the train.

He also assured passengers that the trains were driven by “very competent” staff and that they were under the supervision of such people.

Rail accidents are rare in Denmark, and one expert suggested one of the train drivers had ended up on the wrong line by overriding a stop signal as the train left a local station. The Gribskov line is not thought to have been updated with an automated safety system.