The US House of Representatives' ethics committee has opened an investigation into Republican congressman Matt Gaetz involving allegations including possible sexual misconduct and illicit drug use.
The panel's Democratic chairman and top Republican said in a joint statement they had become "aware of public allegations" that Gaetz, one of former President Donald Trump's most prominent supporters in Congress, may have violated "House rules, laws or other standards of conduct".
Gaetz is being investigated by the US Justice Department for possibly violating sex-trafficking laws by paying travel expenses for a 17-year-old female with whom he was romantically involved, according to a US law enforcement source who spoke on condition of anonymity.
The Florida congressman has not been charged with any crimes and has repeatedly denied wrongdoing.
The ethics committee leaders said the allegations included whether Gaetz "may have engaged in sexual misconduct and/or illicit drug use, shared inappropriate images or videos on the House floor, misused state identification records, converted campaign funds to personal use, and/or accepted a bribe, improper gratuity, or impermissible gift".
Gaetz's office responded in a statement: "Once again, the office will reiterate, these allegations are blatantly false and have not been validated by a single human being willing to put their name behind them."
Speaking at a "Save America Summit" at the Trump National Doral resort in Miami on Friday, Gaetz said "wild conspiracy theories" were being used to smear him.
"I'm built for the battle, and I'm not going anywhere," Gaetz said.
Earlier, Representative Adam Kinzinger became the first congressional Republican to call for Gaetz to quit his post.
"Matt Gaetz needs to resign," Kinzinger, an outspoken critic of Trump, said in a Twitter post on Thursday night.
A friend of Gaetz, a former Florida county tax collector named Joel Greenberg, is expected to plead guilty in a sex trafficking and fraud case in a Florida federal court, officials said on Thursday.
Greenberg's lawyer Fritz Scheller told reporters on Thursday, "I'm sure Matt Gaetz is not feeling very comfortable today."
Scheller may have been referring to media reports that Greenberg could co-operate with federal investigators in their probe of Gaetz.
Two New York criminal defence lawyers, Marc Mukasey and Isabelle Kirshner, on Friday confirmed they had been hired by Gaetz.