A SHOOTING in Utrecht has left one person dead and several injured after a "terrorist" opened fire on a tram as cops confirmed shootings at "multiple" locations and all the city’s mosques were put on high alert.

Heavily armed police are surrounding a building near to the scene of this morning's attack where they believe the suspect is holed up - as the city's terror threat is raised to its highest ever level.

The shooting comes just three days after New Zealand suffered a devastating massacre after a gunman stormed two mosques killing 50 people.

"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."

He declined to comment on the number of injured or what condition the victims were in.

Previously he said the incident "appears to be a terrorist attack" as the government raised its terror threat to 5 until 6pm this evening.

Heavily armed guards have increased their presence at Amsterdam's Schiphol airport and other transportation hubs as children in Utrecht have been urged to stay inside schools with the doors closed.

Witnesses told local media that a man pulled out a weapon and started shooting several people at Utrecht's 24 October Square tram station this morning.

Jimmy de Koster told RTV he saw a woman lying on the ground who shouted: "I have done nothing".

“I came from my job when it happened," he said. "I was standing at the traffic lights on 24 Oktoberplein and I saw a woman lying down, I think she is between 20 and 35 years old.

"At that time I heard pang pang pang three times. Four men walked very fast towards her and they tried to drag her away and then I heard pang pang pang again and those guys let go of that woman again. Very chaos there. "

Conflicting reports have suggested there was more than one perpetrator, but these are yet to be confirmed by police or security forces.

One woman was seen being "hit in the chest by one of more bullets", while three others were said to have been resuscitated on the tram and in the street, ad.nl reported.

Emergency helicopters swooped on the scene and anti-terror cops and military police continue to maintain a heavy presence across the city, gelderlander.nl reported.

In their latest tweet, Utrecht police wrote: "Following the shooting incident of 24 Octoberplein, the municipality of Utrecht, the police and the Public Prosecution Service advise everyone to stay in Utrecht until more is known. New incidents are not excluded. The police are currently looking for the perpetrator".

24 Oktober Square in the city's Kanaleneiland district is a stop on the Utrecht express tram, not far from the city centre. The entire tram network has now been halted as investigations continue.

The US Embassy tweeted to American citizens to "avoid the area, follow local emergency instructions and monitor local news".

More to follow...