Scores of jihadist fighters from Europe who streamed to Syria to join Islamic extremist rebels have begun returning home, where some are suspected of plotting terror attacks, according to U.S. and European intelligence and security officials.

Authorities in the U.K. and France recently made several terror-related arrests of individuals suspected of links to Syria.

"They're real committed jihadists," a senior U.S. intelligence official said. "The concern is that we're at the very early stages of this."

For the U.S. and Western countries, the returning jihadists pose the biggest long-term concern of the Syrian civil war, the official said. Governments are rushing to counter the new terror threat.

"We monitor very closely people seeking to travel [to Syria]—and also people traveling back—because of the potential risk they may pose upon their return to the U.K.," said Britain's security minister James Brokenshire.