British police raced on Saturday to track down whoever planted a bomb on a packed London Underground train, as authorities raised the national threat level to maximum, meaning another attack may be imminent.

Friday morning’s blast, which left 29 people injured, was Britain’s fifth terror attack in six months and was claimed by the Daesh 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 Daesh group.

In a statement Friday, Daesh 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.

Source: Khaleej Times