There is growing evidence that ISIS is experimenting with chemical weapons as the number of foreign fighters hits a new high, according to current and former government officials.

Photos taken by the Kurds in northern Iraq last summer and fall and reviewed by Fox News show burns and blistering on the skin that a source on the ground there said are consistent with the use of chemical agents. The agents were described as "odorless, colorless and absorbed through the clothing," causing burns or illness hours later.

"I think it's, could be a perfect testing ground," former FBI intelligence officer Timothy Gil Sr. said. "They (ISIS) were particularly interested in using these chemicals in confined space environments, soft targets like shopping malls and movie theaters."

Gill who is doing his own independent research for an upcoming international conference on weapons of mass destruction said there is a disturbing trend, that ISIS is using safe havens in Syria, Iraq and potentially Libya as laboratories to learn how chemical agents impact the battlefield, with the goal of sharing their expertise via social media with ISIS followers outside the region.

"It's enough to use in a crude weapon that could really push the panic button," Gill said. "This now provides an opportunity to say 'this is working, we're now going to push this out to any potential lone wolf, lone actor that may have a background in chemistry.'"

A doctor who was in northern Iraq last year and asked not to be identified for security reasons, said he treated Kurdish fighters whom ISIS used as "lab rats for WMD," adding that the variety of burns and illnesses over several weeks suggested to him that "mustard gas, precursors, as well as neurotoxic acids" were being tested.

A U.S. government source said there is reason to doubt ISIS is experimenting with rudimentary chemical agents to instill fear rather than inflict mass casualties.

While ISIS tries to broaden its use of unconventional weaponry, the terror group also has broadened its appeal. A US intelligence official confirms to Fox News the number of foreign fighters has hit a new high -- more than 36,000 from 120 countries since the conflict began in 2011, including at least 6,600 from Western countries.

A year ago, at the worldwide threat hearing on Capitol Hill, where the U.S. intelligence community publicly presented its global view on terrorism, the Director of National Intelligence, James Clapper said that since the conflict began, more than 20,000 foreign fighters from 90 countries had travelled to the region.  A key figure is the number of countries affected, with the extremist ideology now drawing followers from 60 percent of the world's nations.

During congressional testimony before the House Armed Services Committee Wednesday, former senior intelligence officials said ISIS is expanding. 

"ISIS has gained affiliates faster than Al Qaeda ever did -- from nothing a year ago, there are now militant groups in nearly 20 countries that have sworn allegiance to ISIS," former CIA deputy director Mike Morell said.  "They have conducted attacks that have already killed Americans."

On the development of unconventional weapons, Morell added, "They have made two things very clear publicly in documents that have come out. One is that if they acquire these weapons they would use and they wouldn't care that the vast majority of those killed were civilians. and they have also provided a religious justification very similar to Al Qaeda’s, in fact I think it is identical to Al Qaeda’s justification for using such weapons."

The Special Presidential Envoy for the Global Coalition to Counter ISIL (ISIS) recently briefed reporters in Washington DC, emphasizing the overall trend line is in the right direction, pointing to progress securing the Turkish border to stem the flow of foreign fighters.

"This is a challenge like we've never seen before," Brett McGurk said, "The world has never seen something like this, upwards of 35,000 now foreign fighters from 100 countries all around the world supercharged by social media and Twitter and everything. 

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.