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Washington • Attorney General Eric Holder called on European nations Tuesday to deal more aggressively with the threat posed by Westerners who have traveled to Syria to join the fighting there.

In a speech to Norwegian diplomats in Oslo, Holder encouraged European countries to pass laws that make it illegal to prepare for or plan an act of terrorism; to conduct undercover operations to identify individuals planning a trip to Syria; to better share information and data with the U.S. and other countries about foreign fighters; and to develop programs to counter radical extremism.

"This is a global crisis in need of a global solution. The Syrian conflict has turned that region into a cradle of violent extremism," Holder said in his speech. "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."

The speech comes amid growing concerns about citizens from the United States and Europe who are traveling to Syria to join the fight against the Syrian government. U.S. officials fear those individuals, who are able to travel without visas between the United States and Europe, could easily return home radicalized and apply terrorist training received while in Syria.

Intelligence officials believe there are roughly 7,000 foreign fighters in Syria, including dozens of Americans, the attorney general said.

In May, a 22-year-old man from Florida carried out a suicide bombing mission in Syria.

American intelligence officials said earlier this week that they have picked up indications that bomb-makers from Yemen-based al-Qaida in the Arabian Peninsula have traveled to Syria to link up with the al-Qaida affiliate there.

In his address, Holder urged European countries to adopt statutes similar to one in the U.S. — and, more recently, in France and Norway — that make it illegal to provide support for terrorists or to plan acts of terrorism.

He also said European law enforcement should follow the lead of the FBI in designing undercover operations to catch individuals before they depart for Syria. He encouraged European nations to do better in sharing information on foreign fighters.