Islamic State (IS) militants have claimed responsibility for the London Bridge terror attack that killed seven people and wounded dozens of others, news agencies are reporting, citing an IS-linked website.
"A detachment of Islamic State fighters executed yesterday's London attack,” Reuters and AFP quoted a June 4 posting on the Amaq media website. Amaq is considered the main mouthpiece of IS.
The report comes as British police on June 4 said they had arrested 12 people in Barking, east London, in the investigation related to the attack that occurred around 10 p.m. local time on June 3.
A police statement did not provide the names of those held, but it said seven were women aged 19-60 and five were men aged 28-55. Sky News reported that one of the men had since been released.
Police said they were searching four properties believed to be related to the attack.
In the incident on June 3, seven people were killed and at least 48 injured after a van veered off the road on London Bridge and struck several pedestrians. Three occupants left the van and attacked several people with knives in the nearby Borough Market nightlife neighborhood.
All three attackers were shot dead within eight minutes of the initial emergency call by police who fired 50 bullets.
The authorities said they will release the names of the three attackers “as soon as operationally possible.”
WATCH: Londoners Flee After 'Terrorist' Attacks
The terror attack was the third in Britain since late March following a similar incident on March 22 near Parliament on Westminster Bridge and a bombing at a concert by U.S. pop singer Ariana Grande in Manchester on May 22.
A charity concert on June 4 was held for victims of the Manchester attack, featuring Grande, Justin Bieber, and other stars.
The "One Love Manchester" concert at the Old Trafford cricket grounds in Manchester was attended by some 50,000 people. It was completed without incident, organizers said.
British Prime Minister Theresa May said "enough is enough" when it comes to tackling terrorism.
She said the country’s counterterrorism strategy would be reviewed. "We cannot and must not pretend that things can continue as they are," she added.
'This Is For Allah'
Further witness accounts of the attack were reported hours after the incident.
Gerard Vowls told the Associated Press he was on the street when he heard a man moan, "I've been stabbed."
Vowls said a bystander stopped to help the wounded man and then he saw the three attackers strike a woman with knives.
"The three guys, yes, they were just stabbing this woman constantly, nonstop the three of them. Just stabbing her from every direction, the three of them around her. Lunging at her," he said. "I heard them say one thing: 'This is for Allah.'"
Many bystanders credited Vowls with saving lives as he attempted to distract the attackers and ran through the neighborhood yelling out warnings.
Rhiannon Owen, a student nurse, said she was using a cash machine when a taxi driver alerted her.
"The taxi driver just swerved toward me and screamed, 'Run! You have to run! They've got a knife!'" she said.
"His face was just like something was so wrong, and I just started running as fast as I could. There was sirens everywhere, people screaming, the glass was smashed in one shop."
'United In Horror'
World leaders offered support following the latest attack.
"In the face of this new tragedy, France is more than ever at Britain's side," French President Emmanuel Macron wrote in a tweet. "My thoughts go out to the victims and their loved ones.”
German Chancellor Angela Merkel said her country stood "firm and resolute" by Britain's side in the fight against terrorism. "We are united in horror and grief, but also in determination," she said.
Russian President Vladimir Putin offered condolences in a telegram to the British prime minister, the Kremlin said.
"The president of Russia expressed his confidence that the buildup of joint efforts to fight forces of terror all over the world should become the common answer on what happened," a statement said.
U.S. President Donald Trump praised the “heroic response of police and other first responders and offered the full support of the United States government in investigating and bringing those responsible for these heinous acts to justice.”
The U.S. president also engaged in a Twitter battle with London Mayor Sadiq Khan, accusing him of downplaying the threat of terrorism.
Hours after the attack, Khan told the BBC, "My message to Londoners and visitors to our great city is to be calm and vigilant today. You will see an increased police presence today, including armed officers and uniformed officers. There is no reason to be alarmed by this."
"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is 'no reason to be alarmed!'" Trump wrote.
A spokesman for Khan said the mayor had "more important things to do than respond to Donald Trump's ill-informed tweet that deliberately takes out of context his remarks."
Lew Lukens, the acting U.S. ambassador to Britain, offered praise for the London mayor on the embassy’s official Twitter account.
"I commend the strong leadership of the @MayorofLondon as he leads the city forward after this heinous attack," he wrote.