UK terrorism threat level raised from ‘severe’ to ‘critical’ – PM May

British PM Theresa May has raised the country's threat level to critical from severe, following the attack today at Parsons Green tube station in south-west London.

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ISIS claims responsibility for London underground blast

May said in a televised statement that armed police and members of the military would be seen on the streets in the coming days.

"For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” she said.

The enhanced threat level comes just moments after ISIS claimed responsibility for detonating an “improvised explosive device” on a tube train during rush hour Friday morning.

Assistant Police Commissioner Mark Rowley said the device did not fully detonate. Experts believe the contraption was some kind of crude “pressure cooker bomb,” similar to that used in the Boston Marathon bombings.

29 people were injured in the attack. However, none of the victims are said to be serious.

Police, in conjunction with MI5, have launched a manhunt for the assailant.

Britain's top counter terrorism officer said on Friday that authorities are making excellent progress in the investigation and has asked permission for the military to get involved in order to free up police resources.

He further said it was routine for ISIS to claim responsibility for the attack, regardless of whether they were involved or not.

London Mayor Sadiq Khan has released a statement regarding the increased threat level.