The top Republican on the Senate Armed Services Committee said Sunni militants are working on the capability to wipe out a city inside the United States.

The Islamic State in Iraq and Syria (ISIS) has long threatened to carry out a catastrophic attack on American soil, with a spokesman recently boasting that the militant group would fly its flag over the White House.

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Sen. James Inhofe James (Jim) Mountain InhofeChamber of Commerce endorses McSally for reelection Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Top admiral: 'No condition' where US should conduct nuclear test 'at this time' MORE (R-Okla.) said ISIS members are “rapidly developing a method of blowing up a major U.S. city and people just can’t believe that’s happening.”

“Yes, it’s really significant what’s happening,” he added during a local television interview on Wednesday.

President Obama has stepped up U.S. air attacks against the group in Iraq after it posted a video online showing the execution of American photojournalist James Foley.

On Thursday, U.S. Central Command announced it had completed six more attacks near the Mosul Dam, making a total of 90 strikes since Obama authorized them on Aug. 8.

Inhofe said that despite the pounding by U.S. forces, ISIS could still inspire followers to attack stateside.

He also slammed the president for not coming up with a strategy to defeat ISIS.

“He's going to have to come up with something that we're going to do because they're holding another hostage in place and the problem is the president, quite frankly, says all these things and he never does them,” Inhofe said.

ISIS is believed to be holding at least four other U.S. prisoners, including journalist Steven Joel Sotloff, who appeared in the ISIS video showing Foley’s execution. The man speaking in that video threatened that Sotloff would be killed next.