LONDON, Sept. 15 -- British Prime Minister Theresa May said Friday that the terror threat level is raised to critical, which means a further terrorist attack in Britain may be imminent.
May said that her government decided to raise the terror threat level from severe to critical, highest possible level, after an explosion took place on a packed rush-hour carriage at Parsons Green subway station on Friday morning in London.
The British government made the decision on assessment of the current security situation in the United Kingdom by an independent organization, she said.
May described the explosion as a "cowardly attack". She said that members of the British military forces were expected to be seen on streets in London.
This was the fourth time that the assessment has been placed at the highest level in the past 11 years, Sky News reported.
The Islamic State's Amaq News Agency said Friday night that the terrorist group claimed responsibility for the subway explosion in London.
The police said the explosion is a terrorist attack, and a massive hunt is under way to identify the man who placed the home-made bomb on the train.
London police urged anyone with information that could help detectives to contact police hotline on 0800 789 321.
The police said that they are undertaking a complicated investigation to identify the man who is responsible for the blast.
The blast left at least 29 people injured, and most of them are being treated in hospitals for burn injuries.
The blast was caused by an improvised explosive device, which failed to properly detonated, police said.
The Friday blast came after three deadly terrorist attacks in London this year -- Westminster Bridge, London Bridge and Finsbury Park, and one in Manchester at the end of a concert.