"In Utrecht there was a shooting at several locations," Netherlands Counter-Terrorism Agency head Pieter-Jaap Aalbersberg told a news conference.

"A lot is still unclear at this point and local authorities are working hard to establish all the facts. What we already know is that a culprit is at large."

Dutch broadcaster NOS said police had surrounded a building where the suspected gunman was believed to be holed up.

Local broadcaster RTV Utrecht quoted a witness as saying he had seen a woman lying on the ground amid some kind of confrontation and several men ran away from the scene.

A reporter for NOS said a white sheet had been placed over a body near the tram where the shooting had taken place.

Utrecht, the Netherlands' fourth largest city, is known for its picturesque canals and large student population. Gun killings are rare in Utrecht, as elsewhere in the Netherlands.

Utrecht police said the tram station stop in a square outside the city centre had been cordoned off. Emergency services were at the scene.

Reuters/eNCA