WASHINGTON/London -  President Donald Trump sparked fury in London and yet another pointed rebuke from Prime Minster Theresa May Friday after he claimed "loser terrorists" behind a botched London train attack were known to British police.

Trump took to Twitter to condemn an attack on a packed London Underground train that injured 29 people, but also appeared to suggest the UK authorities had dropped the ball.

"Another attack in London by a loser terrorist," the US president tweeted. "These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!" Trump said, appearing to reveal undisclosed intelligence from the headquarters of London's Metropolitan police force.

That electronic broadside brought a terse public response from May, who warned the US president and others not to speculate. "I never think it's helpful for anybody to speculate on what is an ongoing investigation," May told journalists in London.

Trump 's latest comments were similarly described as "unhelpful" by London's Metropolitan Police. Nick Timothy, May's former chief of staff, also branded the US leader's tweet "unhelpful."

"True or not - and I'm sure he doesn't know - this is so unhelpful from leader of our ally and intelligence partner," Timothy wrote on Twitter.

Trump 's keenness to underline a series of UK terror attacks, virtually in real time, has led to repeated outcry across the Atlantic that has helped indefinitely delay his much-vaunted state visit to Britain .

In May, British officials were furious after their US counterparts leaked shared material about an investigation into a bomb attack at a concert by pop star Ariana Grande in Manchester which left 22 dead.

The bomber's identity and details of the probe leaked to US media before British officials felt ready to disclose them.

After an attack in London last June, Trump lashed London's Muslim mayor Sadiq Khan for telling the public there was "no reason to be alarmed" over armed police on the streets.

On that occasion, Prime Minister May was forced to upbraid Trump , saying "I think Donald Trump is wrong in what he said about Sadiq Khan in the aftermath of the London Bridge attack."

In March, the White House left UK spooks apoplectic when it cited unproven media reports that President Barack Obama had asked Britain 's signals intelligence agency, GCHQ, to monitor then-candidate Trump in order to "make sure there were no American fingerprints."

The normally tight-lipped agency called the allegations "nonsense," adding that "they are utterly ridiculous and should be ignored."

Trump on Friday also used the London train attack to renew calls for his controversial ban on travellers from several predominantly Muslim countries - a rallying cry of his right-wing base who have recently been angered by his support for an amnesty for some migrants.

"The travel ban into the United States should be far larger, tougher and more specific - but stupidly, that would not be politically correct," the president said in a series of tweets. "Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!" he added, without elaborating.

Panic after train blast

Witnesses reported seeing a "wall of fire" and passengers with facial burns and hair coming off at Parsons Green station in west London after the explosion on the train.

"At 8:20 this morning at Parsons Green station there was an explosion on a Tube train. We now assess that this was a detonation of an improvised explosive device," police counter-terror chief Mark Rowley said.

Rowley said most of the injuries were due to "flash burns", while others were wounded by the stampede as passengers ran out of the station in panic.

Twitter user @Rrigs, who posted pictures of a white bucket smouldering on the train, said: "Explosion on Parsons Green District Line train. Fireball flew down carriage and we just jumped out open door".

The bucket looked like the type used by builders and there appeared to be cables coming out of it.

One local resident, Charlie Craven, who was on his way to the station, said he heard a "massive bang". "I saw an orange sort of fireball encompassing the whole Tube coming towards you," he told AFP.

Witness Lauren Hubbard described it as "a wall of fire".

The National Health Service said 18 people were taken by ambulance to hospital, while four others made their own way to hospitals.

The London Ambulance Service said none of the victims "are thought to be in a serious or life-threatening condition".

The MI5 intelligence service is assisting investigators and police said Londoners should expect extra security measures over the weekend.

The Islamic State group claimed responsibility for the bomb attack. "The bombing on a metro in London was carried out by a detachment of the Islamic State" group, it said in a statement published by its Amaq propaganda agency.

"There is a manhunt under way as we speak," Mayor of London Sadiq Khan told LBC radio. He urged Londoners to remain "calm and vigilant".

"As London has proven again and again, we will never be intimidated or defeated by terrorism," he said.

Prime Minister Theresa May said her thoughts were with the injured and emergency services had responded "swiftly and bravely". She later chaired an emergency cabinet meeting.

Armed police and sniffer dogs could be seen on the train and around the station, which is set in a leafy suburb of southwest London popular with well-off commuters and filled with chic cafes.

The station was closed, as well as an entire section of the District Line where it is located and police urged people to stay away from the area.

Local residents and businesses rallied together with businesses offering tea and the use of their toilets to people unable to get home.

The incident is the fifth terror attack in six months in Britain since March, when a lone attacker mowed down pedestrians and stabbed a police officer outside the British parliament.

Passengers described chaotic scenes at the station in the normally quiet part of west London.

Louis Hather, 21, was travelling to work and was three carriages down from where the explosion took place. "I could smell the burning. Like when you burn plastic," he told AFP. Hather saw a woman with burns being stretchered off. He was trampled on as passengers stampeded out of the station and his leg was badly cut and bruised.

Sally Faulding, a 51-year-old teacher, said: "People were falling over each other."

Richard Aylmer-Hall, 52, told the Press Association: "There was panic, lots of people shouting, screaming, lots of screaming".

A total of 35 people have been killed in four previous attacks in London and Manchester this year.

Three of those involved a vehicle ploughing into pedestrians.