File photo of US Attorney General Eric Holder
US Attorney General Eric Holder called on Tuesday for cooperation with Europe to stem the "grave threat" of extremists travelling to Syria, saying dozens of aspiring fighters were being investigated by Washington.
On a trip to Oslo, Holder highlighted the urgency in clamping down on US and European nationals seeking to join extremist organisations in Syria and Iraq.
"This is a global crisis in need of a global solution," he told journalists.
"The Syrian conflict has turned that region into a cradle of violent extremism. But the world cannot simply sit back and let it become a training ground from which our nationals can return and launch attacks. And we will not."
U.S. intelligence officials estimate that of some 23,000 violent extremists operating in Syria, more than 7,000 are foreign fighters, including dozens of Americans.
When questioned about the number of US nationals being investigated for their participation in the conflict, Holder said there were "less than a hundred".
He said the ability of European and American citizens to travel visa-free between their two continents meant that "the problem of fighters in Syria returning to any of our countries is a problem for all of our countries.
"In the face of a threat so grave, we cannot afford to be passive."
Holder and Norwegian Justice Minister Anders Anundsen discussed a new plan to counter radicalisation presented by Norway, a country with one of the highest rates of nationals per capita who have travelled to fight in the Syrian conflict.
According to the Norwegian Police Security Service (PST), some 40 to 50 people with links to Norway have joined in fighting against Syrian President Bashar al-Assad.
Holder said investigative and prosecutorial tools were needed to preempt the problem.
"If we wait for our nations' citizens to travel to Syria or Iraq, to become radicalised, and to return home, it may be too late to adequately protect our national security."