(CNN) -- Attorney General Eric Holder urged other countries to enact new criminal laws to help prevent possible terrorist attacks from returning Syrian fighters.
In a speech in Oslo, Norway, on Tuesday, Holder highlighted the threat from domestic extremists such as a lone terrorist who three years ago bombed government buildings in the Norwegian capital and gunned down people at a youth camp, killing 77 people in all.
Holder said the United States, along with Norway and other European countries, face similar danger from "violent extremists fighting today in Syria, Iraq or other locations" who "may seek to commit acts of terror tomorrow in our countries as well."
U.S. intelligence estimates that nearly 7,000 foreign fighters have traveled to Syria, including dozens from the United States.
The issue of Syrian fighters from Western countries is dominating Holder's trip this week to Europe, which includes a meeting in London of attorneys general from the U.S., UK, Canada, Australia and New Zealand.
Open visa access among European countries and the U.S. means "the problem of fighters in Syria returning to any of our countries is a problem for all of our countries," Holder said in his speech, excerpts of which were provided in advance by the Justice Department.
"The Syrian conflict has turned that region into a cradle of violent extremism," he said. "But the world cannot simply sit back and let it become a training ground from which our nationals can return and launch attacks."
To combat the threat, Holder is calling on countries to pass laws to criminalize the preparatory steps that suspects often take before an attack, and to allow police to conduct undercover investigations.
The U.S. has a law that makes it a crime to provide "material support" to terrorists, including supplying money or weapons, or helping to plot an attack. Similar laws are now on the books in Norway and France.
Holder also cited the FBI's success in using undercover sting operations, which have drawn controversy in the U.S. but have been successful in prosecuting dozens of suspects who had admitted to plans to commit terrorism. Many countries don't allow such operations.
But Holder said the use of such operations could help countries thwart attacks. He said the U.S. has already used these tools to carry out prosecutions of people who sought to travel to join the fight in Syria.
"These operations are conducted with extraordinary care and precision, ensuring that law enforcement officials are accountable for the steps they take and that suspects are neither entrapped nor denied legal protections," Holder said.
He also called on countries to share information with Interpol and one another about their nationals who try to travel to Syria to fight and those who return.
And he urged countries to come up with counter-radicalization programs that try to reach communities where young people may be exposed to extremists.
"We must seek to stop individuals from becoming radicalized in the first place by putting in place strong programs to counter violent extremism in its earliest stages," Holder said.