Republicans leaders -- past and present -- reflected on the somber anniversary of Sept. 11, 2001, on a radio program that aired Sunday, some saying they believe New York City and the United States are more vulnerable now to terrorism than at any point since the attacks on the Twin Towers.

"New York is still the world's biggest stage" for an attack, former NYPD Commissioner Ray Kelly said.

"This threat remains constant. I don't see it having diminished," he told 970/AM host John Catsimatidis. "We're going to be in this battle for a long time to come. That's the unfortunate reality. . . . It's a movement more so than just a revolutionary force."

Kelly said one aspect of terrorism that has changed since 2001 is the use of social media -- especially by the Islamic State -- as a recruitment tool and megaphone for messages.

Former Mayor Rudy Giuliani said he feels the same emotions -- anger and a sense of loss -- as he did 14 years ago and warned that the Islamic State, al-Qaida and Iran now have the United States in their crosshairs.

"We're maybe even at more risk now than we were then," he said, adding in a shot to current elected officials: "It seems like we haven't learned our lessons from the past."

Giuliani condemned the White House's nuclear deal with Iran. He said President Barack Obama's "state of denial" over the dangers posed by Iran is similar to the country's mindset toward Osama bin Laden before the 9/11 attacks on lower Manhattan.

Rep. Pete King (R-Seaport) said he remembers that former President George W. Bush showed determination in the days following 9/11 and "got strength" from the police officers, firefighters and construction workers he saw at Ground Zero.

Former Gov. George Pataki, a candidate for the Republican presidential nomination, said the story of 9/11 must be preserved and retold to "future generations," including children who were born since.