Hundreds of U.S., British and Canadian citizens have reportedly been deemed a "very serious threat" on Sunday after training and collaborating with the Islamic State of Iraq and Syria group of extremists.

Chairman of the House Intelligence Committee, Mike Rogers, estimates over 500 people from the U.K. have trained with ISIS, as well as hundreds more from Canada. Although there's a considerable less amount of U.S. citizens training with ISIS, concerns have risen following the recent deaths of two U.S. foreign fighters in Syria.

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All these foreign fighters are a huge threat because they possess passports that allow them to travel in and out of countries, and could potentially create extremist attacks in the U.S., U.K. or Canada.

It's also difficult to be able to determine exactly who's a foreign fighter.

"I'm very concerned because we don't know every single person who has gone and trained and learned how to fight," Rogers said Sunday.

There are many cases confirming U.S. citizens fighting for ISIS overseas. One American foreign fighter went home to the U.S. to visit his family, and as he returned back overseas, it was later reported that he killed himself due to suicide bomb. That sort of extremist attack could have easily happened in the United States.

The Department of Homeland Security, or DHS, said on Friday they'll begin implementing "additional security measures" in an effort to catch any foreign fighters that are known to have contributed anything towards militant extremist groups.

For confidentiality purposes, the full extent of those security measures have not been disclosed, but according to DHS Secretary Jeh Johnson, DHS have already taken numerous steps including increasing security at airports overseas with any direct flights going to the United States. They'll also be implanting measures in an effort to "track foreign fighters who travel in and out of Syria."

The U.S. government will reportedly address the threat of foreign fighters vigilantly. President Barack Obama said that he doesn't have a strategy against ISIS yet, but doesn't rule out the possibility of the expansion of more airstrikes. 

TagsISIS, Islamic state of iraq and syria, U.S., U.K., Foreign fighters, Barack Obama, Mike Rogers, Department of Homeland Security, threat