Leaks of the investigation into the Manchester attack to the US media were “reprehensible” and will be stopped, the top US diplomat in the UK has said.
Lewis Lukens told the BBC he condemned them “unequivocally” and would take action to identify those responsible.
It comes as police described the eight arrests made since Monday night’s bombing, in which 22 adults and children were killed, as significant.
They also say items seized in raids are believed to be “very important”.
On Wednesday, the New York Times outraged British police and government officials when it published photos appearing to show debris from the attack.
They included bloodstained fragments from the bomb and the backpack used to conceal it.
Greater Manchester Police were said to be “furious” and said they would stop sharing information with the US.
Its chief constable Ian Hopkins said the leak undermined the investigation and had distressed families “already suffering terribly with their loss”.
The New York Times newspaper has defended its decision to publish the pictures, saying they were “neither graphic nor disrespectful of victims”.
But in a BBC interview, acting US ambassador to the UK Mr Lukens said: “We have heard the message loud and clear from Her Majesty’s government and we agree with their concerns and we’re determined to take action.
Intelligence co-operation across the Atlantic “keeps both of our countries much safer” but it was “up to the UK ultimately to determine how we work together on this”, he told BBC Radio 4’s World At One.
“These leaks are terrible, and again let me just say in the strongest possible terms that we condemn them and we are determined to investigate and to bring appropriate action.”
Prime Minister Theresa May has said she will make clear to US President Donald Trump that shared intelligence must remain secure at a NATO summit in Brussels.
By Dominic Casciani, BBC home affairs correspondent
The police decision to stop sharing information specifically about the Manchester attack with their security counterparts in the US is a hugely significant move and shows how angry British authorities are.
The information from the crime scene wasn’t shared on a whim: the British and Americans have a lot of shared world-leading expertise in improvised explosive devices and scientists would be discussing whether the Manchester device tells them something new that could, ultimately, track down a bomb-maker.
Other sharing will continue. The UK and US share a vast amount of information about terror and espionage threats – its a tight-knit network that also encompasses Canada, Australia and New Zealand.
That system is based on trust and the “control principle”: if a piece of intelligence is shared, the receiving nation has no right to further disseminate it without permission.
Police believe Manchester-born suicide bomber Salman Abedi, from a family of Libyan origin, acted as part of a network.
Whitehall sources say 22-year-old Abedi was among a “large pool of people whose risk was kept under review” by security services.
The bomber’s older brother Ismail, 23, is among the eight men arrested. A woman was also arrested but has since been released.
Raids involving controlled explosions have been carried out at flats in the city centre and at an address in the Moss Side area of Manchester.
In other developments:
- Armed officers are to patrol trains nationwide for the first time
- A possible suspicious package was declared safe after army bomb disposal experts were called to a street in Hulme, near Manchester city centre
- UKIP’s Suzanne Evans said Theresa May had to take “some responsibility” for the Manchester bombing
- Manchester City and Manchester United have jointly pledged £1m to an emergency fund set up to support the victims
Meanwhile, the Queen has been to the Royal Manchester Children’s Hospital to meet some of the injured, and members of the emergency services.
During her visit, she paid tribute to Manchester and the “extraordinary” way the city had responded to the attack.
Abedi detonated his “nuts-and-bolts” bomb at the end of an Ariana Grande gig as children, teenagers and others were making their way out of Manchester Arena’s auditorium.
Others, including parents, were waiting in the foyer to pick up family and friends when the bomb went off. The blast left 22 people dead, and 116 injured.
Earlier, a minute’s silence fell over offices, public squares and in homes as people honoured the victims.
In Manchester’s St Ann’s Square, a perfectly-observed silence was followed by applause, cheers and a spontaneous crowd rendition of the Oasis song Don’t Look Back in Anger.
Who are the victims?
The latest victims to be named are Wendy Fawell, 50, from Otley, west Yorkshire and Eilidh MacLeod, a 14-year-old from Barra in the Outer Hebrides, and 19-year-old Courtney Boyle.
The youngest known victim so far is eight-year-old Saffie Roussos.
An off-duty Cheshire police officer Elaine McIver was also among the dead.
Colleagues lay flowers in her memory, and tweeted: “We will not let evil win”.
Of the 116 injured, 75 remain in hospital. Of those 23 are in critical care, five of them children.