Theresa May is telling Nato allies that the Manchester attack underlines the need for the alliance to do more in the fight against terrorism.
The Prime Minister arrived in Brussels for a summit with Nato leaders in which she will urge them to "take responsibility" and increase their defence spending.
With Donald Trump attending his first Nato summit, Mrs May will back a call by secretary general Jens Stoltenberg for the alliance to join the US-led coalition against Islamic State (IS).
But Mrs May will also warn Mr Trump that the "special relationship" could be undermined because of the persistent leaking of intelligence material relating to the Manchester atrocity.
As she arrived in Brussels, Mrs May said the attack by suicide bomber Salman Abedi had targeted "innocent and defenceless children and young people".
She said: "That attack, I think, shows why it is important for the international community including Nato to do more in our fight against terrorism and that is what I am going to be pushing for today.
"I am also going to be pushing the UK's agenda on burden-sharing and we are proud, as the UK, that we meet the target of spending 2% of our GDP on defence and 20% of our defence budget on equipment.
"And other nations must be prepared to take responsibility and that includes more investment in defence."
Police involved in the Manchester investigation have stopped sharing intelligence with counterparts across the Atlantic because of repeated leaks to the US media.
Mrs May said: "On the issue of the intelligence-sharing with the USA, we have a special relationship with the USA, it is our deepest defence and security partnership that we have.
"Of course, that partnership is built on trust. And part of that trust is knowing that intelligence can be shared confidently and I will be making clear to President Trump today that intelligence that is shared between law enforcement agencies must be shared securely."
Following the meeting, Mrs May will fly to join other leaders from the G7 group of leading industrialised nations for their annual summit, being held in Taormina, Sicily.
However, a senior Government source said she was expected to cut short her stay in light of events in Manchester, returning to the UK on Friday evening, missing the second day on Saturday.
Earlier, Mr Stoltenberg expressed confidence that the allies would agree to join the counter-IS coalition.
The move is seen as largely symbolic as he made clear they would not be taking part in combat.
"It's totally out of the question for Nato to engage in any combat operations," he said.
Nato already provides surveillance planes to support anti-IS operations as well as training officers in Iraq.
A UK Government source said the alliance signing up to the coalition would send a "strong political message" of its commitment to the counter-IS cause.
The source said that would also enable the alliance to build up the capabilities of partner states in the region such as Jordan, where Britain is volunteering to run a pilot project co-ordinating counter-terrorist action.