U.S. Representative Adam Kinzinger has called for the resignation of fellow Republican Matt Gaetz, a staunch ally of former President Donald Trump who faces a federal probe into a possible relationship with an underage girl.

ā€œMatt Gaetz needs to resign,ā€ Kinzinger, an outspoken critic of Trump, said in a Twitter post on Thursday night.

Kinzinger is the first Republican member of Congress to call on Gaetz, a congressman from Florida, to step down.

Gaetz is being investigated by the U.S. Justice Department for possibly violating sex-trafficking laws by paying travel expenses for a 17-year-old girl with whom he was romantically involved, according to a U.S. enforcement source who spoke on condition of anonymity.

Gaetz has not been charged with any crimes and has denied wrongdoing.

He was scheduled to speak on Friday at the ā€œSave America Summitā€ taking place at the Trump National Doral resort in Miami.

(Reporting by Doina Chiacu; Editing by Scott Malone and Chizu Nomiyama)