London - More than 20 passengers were injured, some suffering severe burns, in an early morning explosion on a London Underground train in what police described as a "terrorist incident," the fifth attack in six months in Britain.What happenedEmergency services were called around 8:20 am (07:20 GMT) after the explosion at Parsons Green station in a leafy southwestern suburb which police counter-terror chief Mark Rowley said had been caused by an "improvised explosive device." Police immediately declared an investigation into a "terrorist incident." One Twitter user who posted pictures of a smouldering white bucket in the carriage described a "fireball" which "flew down (the) carriage." Prime Minister Theresa May was preparing to chair an emergency cabinet meeting while her foreign minister, former London mayor Boris Johnson, appealed for calm as did current mayor Sadiq Khan.Months of attacksBritain has suffered months of repeated attacks, killing 35, since a lone attacker in March mowed down pedestrians and stabbed a police officer outside parliament before police shot dead Khalid Masood, a Muslim convert with a history of violence. He is believed to have acted alone. In May, Salman Abedi, a Briton of Libyan origin, blew himself up outside a pop concert in Manchester, killing 22 - a third of them children. Islamic State claimed responsibility as well as for a June attack near London Bridge which left eight dead. Police shot dead three attackers after they went on the rampage.Who was behind the bombing?There were no immediate claims of responsibility. US President Donald Trump blasted "loser terrorists" behind the latest attack as "sick and demented people" and blamed the internet as the perpetrators' main recruitment tool.VictimsThe National Health Service put the injured toll at 22. The London Ambulance service said 18 people were taken to four London clinics and the NHS added four passengers made their own way to hospital. Witnesses reported several passengers on the train had been "badly burned" by the blast. One journalist at the scene reported some victims' "hair coming off."