WASHINGTON: US 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.

"Matt Gaetz needs to resign," 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 US 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 US 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 "Save America Summit" taking place at the Trump National Doral resort in Miami.