A rookie firefighter accused of scouring the Internet in search of sex with teenaged girls was arrested on federal charges after a cyber-vigilante group allegedly snared him in a sting operation.

Ryan Hogan, 24, is accused of trying to rendezvous with a 14-year-old sex partner over the Internet – and sending her a live-feed video of himself masturbating, wearing only an FDNY shirt with the Engine Co. 237 logo.

A firefighter for 15 months, Hogan pleaded not guilty at his arraignment yesterday, where officials immediately suspended him without pay.

“We are shocked and appalled at the charge,” a Fire Department spokesman said, adding that Hogan could face termination if convicted.

The stocky blond smoke-eater’s downfall began on March 8 when he allegedly struck up an online chat with someone using the screen name “Cuteashley 4U1990” who claimed to be 14 years old – and told the “girl” he was “hoping to get some.”

But the firefighter didn’t know his new pal belonged to Perverted Justice, an Internet watchdog group, according to court papers filed by Assistant U.S. Attorney Lawrence Ferazani Jr. Over the next two days, Hogan repeatedly e-mailed Cuteashley – sending some of the graphic messages from a computer at Engine Co. 237 in Bushwick – and laid plans to have sex at her house while her mom was away, the feds said.

The accused pedophile even drove by the address Cuteashley gave him, according to court papers.

Hogan then allegedly transmitted the webcam video of him masturbating after Cuteashley swore she wasn’t law enforcement.

The accused pervert showed up for court yesterday in another FDNY shirt – this one commemorating 9/11 with the slogan, “In memory of our fallen brothers. May they rest in peace.”

But he covered up in his father’s long-sleeved shirt and hat after he was released on $250,000 bail.

The city’s Department of Investigation conducted the probe after Perverted Justice alerted fire officials. The charges carry a possible sentence of between five and 30 years behind bars.

Hogan is the second city employee in a week to be nabbed for allegedly using the Internet to try to lure underage girls into sex.