The London train explosion is the latest of 5 UK terror incidents in 2017

The blast of an improvised bomb Friday on a London Underground train brings the number of terror incidents in the United Kingdom this year to at least five.

The explosion, which injured at least 23 people during the morning rush, is being treated as terrorism, London's Metropolitan Police said.

"My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident," UK Prime Minister Theresa May said in a message tweeted by Downing Street.

Here are the five terror events that have unfolded in 2017 in the UK:

March 22: Westminster Bridge vehicle attack and stabbing

Khalid Masood drove a rental car into pedestrians on London's Westminster Bridge before exiting the vehicle and stabbing a police officer to death at the entrance to Parliament, police said. Four other people died as a result of the rampage, and 50 people were injured.

Masood, 52-year-old British man, was shot dead by other officers.

He acted alone, police said, but may have been inspired by ISIS.

May 22: Manchester bombing

A bomb exploded just outside Britain's Manchester Arena as people were leaving an Ariana Grande concert. Twenty-two people, including children, were killed in the terror attack.

The bomber, Salman Abedi, a 22-year-old a British national of Libyan descent, also was killed, police said. ISIS claimed responsibility for the attack.

June 3: London Bridge and Borough Market attack

Three men drove a van into pedestrians on London Bridge before going on a stabbing rampage at bars at nearby Borough Market.

The attack killed eight people and injured more than 40 others. The attackers were shot dead by police.

Police named the attackers as Khuram Shazad Butt, 27; Rachid Redouane, 30; and Youssef Zaghba, 22.

ISIS claimed responsibility for the attack, though the group provided no evidence for its involvement.

Butt is believed to have associated with the outlawed radical Islamist group al-Muhajiroun, co-founded by notorious hate preacher Anjem Choudary. Butt was considered a potential threat to British security and was still under active investigation at the time he carried out the deadly assault, counterterrorism sources told CNN.

June 19: Van hits pedestrians outside mosque

A man drove a van into a group of pedestrians who had attended late-night Ramadan prayers at London's Finsbury Park Mosque, police said, killing one man and injuring 11 other people.

Police arrested Darren Osborne, 47, a resident of Cardiff in Wales, on suspicion of murder and terrorism offenses.

September 15: Blast on London Underground train

An improvised bomb detonated on London Underground commuter train at about 8:20 a.m. local time at the Parsons Green station in central London, police said.

At least 23 people were treated at hospitals for injuries, most apparently the result of flash burns, Metropolitan Police Assistant Commissioner Mark Rowley said.

"We now assess that this was a detonation of an improvised explosive device," he said at a news conference.

Asked by a reporter if police had any information about the perpetrator, Rowley said: "It's a live investigation."