David Cameron and Barack Obama have announced joint plans to fight "the poisonous ideology" of Islamist extremists.
Speaking after two days of talks at the White House, the leaders said they are setting up a new group to exchange information and expertise to tackle the terrorist threat.
"We face a poisonous and fanatical ideology that wants to pervert one of the world's major religions, Islam, and create conflict, terror and death. With our allies, we will confront it wherever it appears," Mr Cameron said at a news conference with Mr Obama.
"We know what we are up against. And we know how we will win."
The announcement comes amid fears about the spread of terrorism after last week's Paris attacks that left 17 people dead and the arrest of more than two dozen people in anti-terror raids in Belgium, Germany and France.
Mr Obama said the attacks in France "underscored again how terrorist groups like al Qaeda and ISIL (Islamic State) are actively trying to inspire and support people within our own countries to engage in terrorism."
"We both recognise that intelligence and military force alone is not going to solve this problem so we are also going to keep working together on strategies to counter violent extremism that radicalises, recruits and mobilises people, especially young people, to engage in terrorism," the President said.
Mr Obama also suggested that European countries needed to make sure their Muslim populations were better "assimilated".
"The US has one big advantage in this whole process ... our Muslim populations, they feel themselves to be Americans," he said.
"And there is, you know, this incredible process of immigration and assimilation that is part of our tradition - that is probably our greatest strength."
Mr Cameron had arrived in Washington with a request for the President to help persuade American technology companies to give governments more access to encrypted communications that terrorists may use to plot attacks.
"As technology develops, as the world moves on, we should try to avoid the safe havens that could otherwise be created for terrorists to talk to each other," Mr Cameron said at the news conference.
"We're not asking for back doors" to access electronic communications, Mr Cameron said.
"We believe in very clear front doors through legal processes that should help to keep our country safe."
The Prime Minister's policy proposals have caused concern on both sides of the Atlantic about the prospect of security efforts encroaching on privacy.
Mr Obama did not take a position on Mr Cameron's proposal, but said it was important to be able to keep tabs on terrorists' use of the internet and social media.
"When we have the ability to track that, in a way that is legal, conforms with due process, rule of law and oversight, then that's a capability we have to preserve," Mr Obama said.
The two leaders also agreed to stage cyber "war games" and establish a joint "cyber cell" to boost both countries' resistance to hack attacks.
And there was also time for them to once again express their admiration for one another.
The President paid tribute to Mr Cameron as a "great friend" and "outstanding partner", while Mr Cameron said the President was a "good friend" who shared the "same values".
