Britain raised its threat level to maximum on Friday and announced troops would be deployed to key sites after a bomb detonated on a packed London Underground train, injuring at least 29 people in an attack claimed by the Islamic State group.

The explosion — Britain’s fifth terror attack in six months — sparked a “wall of fire” that left passengers with burns and caused a stampede of panicking people in which some were trampled.

Twelve hours after the blast at Parsons Green station in southwest London, Prime Minister Theresa May announced the national threat level would be raised to “critical”, meaning another attack may be imminent.

She said military personnel would take over guard duty at certain closed “protected sites”, freeing up 1,000 police officers to be deployed on the transport network and on streets across Britain.

The country was last on critical alert after the bombing at a concert in Manchester in May, which was also claimed by the IS group.

In a statement Friday, IS said a “detachment” had carried out Friday’s attack in London.

No-one has yet been arrested over the bombing, but anti-terrorism police chief Mark Rowley said the investigation was making “really good progress”.

“We’re chasing down suspects,” he told reporters.

“Somebody has planted this improvised explosive device on the Tube. We have to be open-minded at this stage about him and potential associates.”

Rowley earlier said most of the injuries were due to “flash burns”, while others were wounded as passengers ran out of the station in panic.

A local resident, Charlie Craven, who was on his way to the station at the time of the attack, 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”.