US President Barack Obama and British Prime Minister David Cameron have pledged a joint effort to curb violence following last week's deadly attacks in France.
Cameron's visit to Washington came days after 17 people were killed in attacks in France, heightening fears in Europe and the US about the spread of violence.
"This is a problem that causes great heartache and tragedy and destruction," Obama said in a joint news conference with Cameron on Friday. "But it is one that ultimately we are going to defeat."
Cameron was blistering in his assessment of those responsible for the French attacks, calling them part of a "poisonous, fanatical, death cult".
"We know what we're up against, and we know how we will win," he said.
Obama said the US would hold a summit in February on countering violent extremism and the threat of fighters returning to their home countries from the war in Syria.
"David and the United Kingdom continue to be strong partners in this work, including sharing intelligence and strengthening border security," he said.
The US and Britain already co-operate closely in global electronic surveillance, and Cameron said the two leaders had agreed to deepen their co-operation on cyber-security.
As Obama and Cameron met at the White House, representatives from their countries were also joining negotiating partners for another round of nuclear talks with Iran.
Negotiators have set a March deadline for reaching a framework that would address the international concerns about Iran's nuclear programme.
Obama has faced mounting calls from Republican critics for tougher new sanctions on Iran, with his opponents saying a debate on more stringent measures could take place in the US Senate within weeks.
Both leaders strenuously urged US Congress to avoid ordering new economic sanctions on Iran in the midst of those negotiations, arguing that doing so could upend the delicate diplomacy.