This undated image posted on Wednesday, Aug. 27, 2014 by the Raqqa Media Center of the Islamic State group, a Syrian opposition group, which has been verified and is consistent with other AP reporting, shows a fighter from the Islamic State group, armed with a knife and an automatic weapon, next to captured Syrian army soldiers and officers, following the battle for the Tabqa air base, in Raqqa, Syria.

WASHINGTON — Defense Secretary Chuck Hagel said Wednesday that more than 100 Americans are fighting with Islamic State militants, the first time the Pentagon chief has put a number on U.S. citizens aiding the terrorists.

Hagel cited the Americans aiding the jihadists to illustrate the threat that the Islamic State poses to the United States.

“We are aware of over 100 U.S. citizens who have U.S. passports who are fighting in the Middle East with ISIL forces,” Hagel told CNN, using the U.S. government’s preferred acronym for the Islamic State. “There may be more. We don’t know.”

U.S. intelligence officials have said that as many as 300 Americans may have joined forces with the Islamic State. They are among thousands of Westerners believed to be fighting alongside the militants.

Hagel spoke after U.S. analysts verified as real a video released Tuesday showing the beheading of reporter Steven Sotloff, the second American journalist slain in such a gruesome manner, following the slaying of James Foley, a video of which was posted Aug. 19.

“It makes you sick to your stomach, but it again reminds you of the brutality and barbarism that is afoot in some places in the world,” Hagel said.

Vice President Joe Biden used stronger words Wednesday during a speech in Portsmouth, N.H.

“When people harm Americans, we don’t retreat, we don’t forget,” Biden said. “We take care of those who are grieving.”

His voice rising to almost a shout, Biden added: “And when that’s finished, they should know we will follow them to the gates of hell until they are brought to justice — because hell is where they will reside. Hell is where they will reside.”

President Barack Obama employed more restrained language in making a similar point.

“Those who make the mistake of harming Americans will learn that we will not forget … that our reach is long and that justice will be served,” Obama told reporters in the Estonian capital of Tallinn.

Obama was to join other allied leaders Thursday and Friday at a NATO summit in Wales.

In the videos released by the Islamic State, the black-shrouded figures who beheaded Foley and Sotloff spoke with a British accent. About 500 Brits are thought to be fighting with the group, along with hundreds more from France, Russia and other European countries.

The Islamic State militants have said the beheadings are their response to U.S. airstrikes that Obama launched against them in Iraq almost four weeks ago.

Hagel made his remarks during an interview at the U.S. Naval War College in Newport, R.I. He said extending the bombing campaign to Syria, the terror group’s hub, is among a number of military options he and other national security leaders are considering, though he ruled out sending U.S. combat troops to the region.

Since the Islamic State fighters began sweeping across Iraq almost three months ago, Obama has dispatched 760 American troops to advise Iraqi forces, set up joint operations centers in Baghdad and Irbil, and to assess the threat.

Under the War Powers Act, Obama is asking Congress for authorization to send 405 more troops to Iraq, with 55 to be pulled out for a net increase of 350. Obama has faced criticism in recent weeks for taking too long to come up with a strategy to counter the militants.

“They control half of Iraq today,” Hagel said Wednesday. “They control half of Syria today. We better take them seriously.”

Hagel urged Congress to approve Obama’s request for $500 million to equip and train moderate Syrian rebels who are combating both the Islamic State fighters and Syrian forces loyal to Syrian President Bashar Assad.

David Goldstein of the Washington bureau contributed.

©2014 McClatchy Washington Bureau. Distributed by McClatchy-Tribune Information Services.