LONDON — Theresa May is angry with the U.S. over leaky intelligence — but the British prime minister also plans to use her second meeting with President Donald Trump to wrest assurances on Russia and NATO.

The U.K. prime minister will fly into Brussels Thursday for her first summit with NATO leaders amid rising tensions at home over leaking of operational intelligence from the police investigation into the suicide bomb attack in Manchester on Monday.

There is palpable fury inside Downing Street and from British security services over the quantity of information finding its way into the public domain, after the name of the attacker and photographs reported to be images of the crime scene taken by British investigators were published Wednesday, despite a rare public warning from Home Secretary Amber Rudd for the leaks to stop.

A senior Whitehall official said U.K. ministers had made clear their outrage to Washington, who they blame for the leaks, in what they describe as an “unacceptable” breach of trust. Defense Secretary Michael Fallon, who will accompany May to Brussels, said the leaks put the counter-terror investigation at risk.

"These images will be distressing to victims, their families and the wider public," the official said. "British officials and ministers are making it clear to their U.S. counterparts that these leaks from within the system are unacceptable."

The row forms an uncomfortable backdrop for May amid heightened security in the U.K. following the worst terrorist attack on British soil in over a decade in which 22 people, including young children, were killed at a pop concert. In a statement Thursday morning, the British prime minister said she would "make clear to president Trump that intelligence that is shared between our law enforcement agencies must remain secure” when she meets him at the summit later in the day.

NATO officials are hoping the summit will bind Trump into the alliance, which he openly criticized on the campaign trail, branding it "obsolete." Secretary General Jens Stoltenberg has set an agenda for Thursday’s summit that addresses the two primary concerns raised by Trump during his presidential campaign: that other NATO member countries are not contributing enough and that the organization is not focused enough on tackling terrorism. A senior European NATO diplomat said — approvingly — Wednesday that Trump had successfully "dictated" that terrorism be near the top of NATO's agenda.

According to a senior U.K. government official briefed on May's summit plan, the prime minister will echo Trump's call for the alliance to do more to tackle global terrorism and will warn NATO leaders they need to spend more on defense to meet their obligations.

Russian threat

While she plans to push NATO on terrorism, May will also risk undermining the U.S. president's professed desire for a rapprochement with Moscow by singling out Vladimir Putin's regime as the second major threat to global security alongside global terror. "Our unity in responding to common threats is our most potent weapon," she will say, according to the official. "We must redouble our resolve to meet the threats to our shared security, whether from terrorism or Russia."

The summit is also an opportunity for May to highlight Britain's commitment to international security cooperation ahead of upcoming Brexit negotiations, set to start next month, which threaten to uproot one of the foundation stones of British national security.

By highlighting the threat posed by Russia, May hopes to win support among her European counterparts but also risks further inflaming relations with Trump, under pressure at home from a federal investigation into possible collusion between the Trump campaign and Moscow. A U.K. government official explained the inclusion of Russia alongside terrorism in the PM’s expected remarks, saying: “The PM has said in relation to Russia, engage but beware, and that remains our position.”

After the summit in Brussels May will fly south to Sicily for a meeting of the G7, where she will lead talks on the threat posed by global terror in a session on Friday afternoon. She will then fly back to the U.K. a day earlier than planned to oversee from Downing Street the security operation ordered after Monday’s attack.

U.K. security services have been placed on high alert, with troops deployed onto the streets and the terror threat level at “critical,” meaning an attack is expected imminently.

Security first

Security has long been a priority for the British prime minister.

In her previous role as home secretary, May said she was in favor of remaining in the EU primarily because it strengthened Britain’s national security.

In a speech just over a year ago she said Brexit would make it harder to share intelligence and would undermine Britain’s influence in constructing European security mechanisms.

Leaving the EU’s institutional security framework risks leaving Britain exposed, and raises questions about the U.K.’s future participation in Europol, the international arrest warrant and defense mechanisms.

Leading U.K. security officials, who spoke to POLITICO on the basis of anonymity, said Britain would look to NATO to fill the gap if it was unable to remain involved, in some form, in the EU’s Commons Security and Defense Policy.

After a difficult week at home, the British prime minister faces a diplomatic tightrope walk abroad.

Britain believes NATO has an important part to play taking on ISIS in Syria and Iraq because of its “operational experience and tools needed to add value.” May will back calls for NATO to formally join the U.S.-led coalition against ISIS by becoming an official member.

While May is committed to the NATO alliance and the U.S. special relationship as the twin cornerstones of U.K. security, May's appearance at the summit comes amid unprecedented anger with Washington, raising question marks over the future of the intelligence-sharing relationship in what would be a major blow for the U.K.

Rob Wainwright, the British head of Europol, hinted at the damage done by the U.S. leaks, tweeting it might have been “two slips” but noting that Britain placed huge emphasis on the U.S. security relationship, especially the “five eyes” intelligence sharing deal with Canada, Australia and New Zealand. “The times I've been told that '5 eyes' partners are so much more reliable than [Europol] for handling sensitive data,” he wrote.

French Interior Minister Gérard Collomb also appeared to breach protocol Wednesday, telling French television that Manchester attacker Salman Abedi had traveled to Syria and that he had “proven” ties to the Islamic State before the U.K. authorities had confirmed this. “Today, we know only what British investigators have told us,” Collomb said.

After a difficult week at home, the British prime minister faces a diplomatic tightrope walk abroad.

David M. Herszenhorn contributed reporting.

This story has been updated to include details of an additional statement from Theresa May.