Two trains collided head-on in Denmark near Hillerød, just north-west of Copenhagen, injuring at least 18 people and leaving five in critical condition.

The crash occurred early Thursday morning at a level crossing, sending both trains heavily damaged and scattering debris, including shattered glass.

All 37 passengers were evacuated, with some flown to hospital by military helicopter.

Authorities have launched an investigation into possible causes, including signalling faults and human error, but say it is too early to draw conclusions, The Guardian has reported.

Officials described the rare collision as something that should not have been possible and said a crisis centre has been set up for those affected.