President Donald Trump on Friday told British Prime Minster Theresa May the United States will continue to help the U.K. thwart terror attacks, the White House said after an attack on a London subway train.

In a telephone call with May, Trump expressed support for those injured in the explosion and "pledged to continue close collaboration with the United Kingdom to stop attacks worldwide targeting innocent civilians and to combat extremism," the White House said.

A home-made bomb exploded on a packed rush-hour train in London's Tube injured 29 people on Friday, police said, in what was being treated as the fifth terrorism attack in Britain this year.

Passengers on board the train heading into the capital fled as fire engulfed a carriage at Parsons Green underground station in West London after the explosion at 8:20 a.m.

Some suffered burns while others were injured in a stampede to escape. The National Health Service said 19 people had been taken to London hospitals, while four others showed up on their own. None was thought to be in a life-threatening condition, the ambulance service said.

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An unverified image on social media appeared to show a white bucket on fire inside a supermarket bag on the floor of one train carriage. The same picture does not seem to show extensive damage to the train.

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Metropolitan Police Assistant Commissioner Mark Rowley said it was an improvised explosive device and most of those injured had "flash burns." The scene was cordoned off as the investigation continued.

"It is very much a live investigation we are following down the lines of inquiry," Britain's top counter-terrorism officer said.

"We now assess that this was a detonation of an improvised explosive device," Rowley told reporters. He said most of the injuries were thought to be flash burns.

Rowley declined to answer whether the authorities knew who was responsible or if the suspected bomber had been on the train, saying it was a live investigation which was being assisted by the intelligence services.

A correspondent for Britain's Sky News tweeted that British security forces have "identified a suspect involved" in the bombing via surveillance footage.

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Pictures taken at the scene showed a white bucket with a supermarket freezer bag on the floor of one train carriage. The bucket was in flames and there appeared to be wires coming out of the top.

"I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," Ola Fayankinnu, who was on the train, told Reuters. "There were phones, hats, bags all over the place and when I looked back I saw a bag with flames."

Another witness, who did not want to be named, said: "It was every man for himself, people were panicking, people just ran over each other. There was a lady with badly burned legs, it was pretty shocking. The woman I was with was just run over — it was a stampede."

Outside the station, a woman was sitting on a pavement with a bandage around her leg, while armed police patrolled. A Reuters witness saw a woman being carried off on a stretcher with her legs covered in a foil blanket.