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Homeland Security increases security for some U.S.-bound flights

The Department of Homeland Security announced Wednesday that it will require increased security on some international flights bound for the United States.

The flights, coming predominantly from Western Europe and the Middle East, will be subject to greater scrutiny of passengers and cargo, the department said, though officials did not spell out exactly what the increased security measures will entail.

Flash Points: Why are foreign fighters gathering in Iraq and Syria?

The move comes amid heightened concern about foreign fighters participating in the violence in Syria and Iraq, where Islamist militants with the Islamic State of Iraq and Syria (ISIS) have been waging a bloody campaign in recent months to seize territory from the government.

U.S. intelligence officials estimate that as many as 7,000 foreign nationals have flocked to Iraq and Syria to join the fight, bringing the total number of ISIS fighters to somewhere between 15,000 and 20,000. Of the 7,000 foreigners there, the FBI estimates that maybe 100 are Americans.

The concern is that the foreigners bearing Western passports could return home to attack their fellow citizens in the West.

"There's still thousands and thousands of foreign fighters in Syria, and it's a focus of mine and I know from my foreign trips that it's a huge focus especially of our European counterparts," said FBI Director James Comey in a recent meeting with reporters. "We're all worried."

Comey suggested the United States was blind to the threat coming out of Afghanistan before 9/11, saying the country must not make the same mistake twice.

"All of us with a memory of the 80s and 90s saw the line drawn from September 11," he said. "We see Syria as that, but in order of magnitude, worse. In a couple respects: far more people going there, far easier to travel to and back from. So there's going to be a diaspora out of Syria at some point and we are determined not to let lines be drawn from Syria today to a future 9/11."

It's important to note that the increased security measures are not related to the July 4 holiday. Officials say there has been no increase in the terror chatter monitored by U.S. intelligence services, and that the added scrutiny was not prompted by any specific threat.