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The 'fireball' which ripped through a London Underground train is now officially being treated by London police as a terrorist act. 

The Ministry of Foreign Affairs says there is no sign of any New Zealanders being injured in the incident.

Terrified commuters have described how they had to "run for their lives" after the explosion on the train in Parsons Green, south London, left several people injured.

The explosion has been called a 'fireball', with passengers saying it shot through the train's carriage. Police are currently searching other stations as well as the train involved. 

British Prime Minister Therea May has responded to the incident, saying her thoughts are with those who are injured as well as the emergency services.

PM: My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.

— UK Prime Minister (@Number10gov) September 15, 2017

Kiwi builder Brian Moore was on the District Line train when he said it lurched to a halt.

"Everyone just... panicked. I just saw a stampede of people... running for their lives... chaos."

Moore, who had been on his way to work, said "it got real crazy, real quick... I've never seen anything like it".

Pictures from the District Line train appear to show a burning plastic bucket stashed in a Lidl carrier bag.

The image appears to show a number of wires protruding out of the top and onto the train carriage floor.

Parsons Green explosion. People reporting that a "Fireball flew down carriage". More updates @BBCBreaking & at 1.30pm BBC1 #ParsonsGreen pic.twitter.com/UoFF4RoAwF

— Michael Gravesande (@OldBlackHack) September 15, 2017

Britain's Foreign Secretary Boris Johnson is warning Londoners not to panic, and assured worried individuals that the Police were investigating.  

A video posted to Twitter appears to show the aftermath of the fireball on the train.

#parsonsgreen pic.twitter.com/0OUV819EtE

— Sylvain Pennec (@sylvainpennec) September 15, 2017

Emergency services are at the scene, and horrified witnesses on social media claimed people were "screaming and running off trains".

Armed police are also said to have been at the scene. 

We are aware of reports on social media RE #ParsonsGreen. We will release facts when we can - our info must be accurate

— Metropolitan Police (@metpoliceuk) September 15, 2017

A witness on Twitter said there were explosions in Parsons Green district line train.

"[A] Fireball flew down carriage and we just jumped out open door."

Another social media user said: "What's happening at Parsons Green, panic on the platform reports of fire or explosion in a bag. No announcements."

Was on #parsonsgreen train when mass of people came rushing at us after 'explosion' on rear carriage. Surreal moment. Adrenaline pumping.

— G@rth (@garthserg) September 15, 2017

A London Fire Brigade spokeswoman said: "We were called at 8.21am (local time) and we have information there was a fire on a train eastbound at the platform at Parsons Green.

"We have got two fire engines there," she said.

The London Ambulance Service also says it is on the scene, tweeting: "We are on scene alongside the Met Police at Parsons Green. More information to follow."