President Donald Trump has said that "deeply troubling" leaks to US media about the Manchester suicide bombing would be investigated, after irate British police stopped sharing information with US agencies.
British Prime Minister Theresa May said earlier she would tell Mr Trump that intelligence shared between their two countries had to remain secure, in a rare public show of dissatisfaction with Britain's closest security ally.
"The alleged leaks coming out of government agencies are deeply troubling," Mr Trump said in a statement released after he arrived in Brussels for a NATO summit, also attended by May.
"I am asking the Department of Justice and other relevant agencies to launch a complete review of this matter, and if appropriate, the culprit should be prosecuted to the fullest extent of the law."
Terror network behind Manchester bombing?
Salman Abedi, a 22-year-old British-born man with Libyan parents, blew himself up on Monday night at the Manchester Arena indoor venue after a concert by US singer Ariana Grande, whose fans are mostly children and teenagers.
The explosion killed 22 people, ranging from an eight-year-old schoolgirl to parents who had come to pick up their children.
With the official threat level raised to "critical", meaning a further attack could be imminent, troops have been deployed to free up police, and armed officers patrolled trains for the first time in Britain.
Police, who believe Abedi was part of a network, are holding eight people in custody in connection with the attack.
"I want to reassure people that the arrests that we have made are significant, and initial searches of premises have revealed items that we believe are very important to the investigation," Manchester's police chief Ian Hopkins said.
Abedi, who was known to security services, had recently returned from Libya, where his father and younger brother were arrested by a militia on Wednesday.
Abedi passed through Istanbul on his way to Europe, Turkish security officials told Reuters, but said they had no records of him entering Syria during his travels as had been suggested on Wednesday by the French interior minister.
Investigation not compromised - UK
Over the past three days several key details of the investigation, including the name of the bomber, first came out in US media, angering British police who feared such leaks risked compromising their investigation.
The decision to stop sharing police information was an extraordinary step for Britain, which is usually at pains to emphasise its "special relationship" with the United States.
"This is until such time as we have assurances that no further unauthorised disclosures will occur," said a counter-terrorism source, who spoke on condition of anonymity.
British authorities did not say that the investigation had in fact been compromised by the leaks.
The attack injured 116 people, of whom 75 were admitted to hospital and 23 remain in a very serious condition, health authorities said.
Queen visits the injured
Queen Elizabeth visited the Royal Manchester Children's Hospital, where some of the casualties have been treated, and was filmed chatting with a girl lying injured on a ward.
The city's two football giants, Manchester United and Manchester City, laid aside their fierce rivalry to jointly donate £1 million to an emergency fund set up to support families affected by the attack. They labelled their initiative 'A City United'.
Pictures published by the New York Times included remains of the bomb and of the rucksack carried by the suicide bomber, and showed blood stains amid the wreckage.
In a statement, the Times defended its decision to publish the images, saying they were "neither graphic nor disrespectful of victims".
"Our coverage of Monday's horrific attack has been both comprehensive and responsible," the newspaper said.