President Obama.

"We have seen Europeans sympathetic to their (militants') cause travelling into Syria and may now travel into Iraq, getting battle-hardened. Then they come back," Obama warned in an interview that aired Sunday on the US broadcaster ABC.

These combatants "have a European passport. They don't need visas to get into the United States," he told "This Week."

"Now, we are spending a lot of time, and we have been for years, making sure we are improving intelligence to respond to that.

"We have to improve our surveillance, reconnaissance, intelligence there. Special forces are going to have a role. And there are going to be times where we take strikes against organizations that could do us harm."