FBI Investigates R. Kelly for Flying Underaged Azriel Clary Over State Lines

9:00 AM PT -- R. Kelly's attorney Steve Greenberg tells us, "Mr. Kelly has not violated the law. Certainly not the Mann Act. The story has flawed factual predicates. Beyond that, we remain unaware of any investigation, anywhere."

R. Kelly allegedly arranged a coast-to-coast trip for Azriel Clary when she was underage, and the FBI is now looking into the trip to see if R. Kelly committed a federal crime ... TMZ has learned.

Azriel, one of the women featured in "Surviving R. Kelly," was flown from Palm Springs to Phoenix to Orlando on May 25, 2015. She flew US Airways, leaving Cali at 6:57 AM and eventually landing in Florida at 4:09 PM.

A source connected to Azriel's family tells us the flights were arranged by a woman who was R. Kelly's assistant at the time.

Here's why it's potentially a big deal. Azriel was only 17 at that time and that means R. Kelly could have violated the Mann Act -- the federal act that makes it illegal to transport a minor across state lines for purposes of sex, debauchery or prostitution.

We're told the FBI just learned about the flights within the last week. As we reported ... the FBI has also been in touch with Joycelyn Savage's father multiple times in the last 18 months -- so it's clear the feds are taking the R. Kelly allegations seriously.

R. Kelly sources tell us the singer believes he did nothing wrong because he claims this signed note from Azriel's mom, Alice ... gave Kelly's assistant consent to care for Azriel.

It's not a great defense, because you can see it's dated Sept. 21, 2015 -- but Azriel made the trip back in May 2015. However, we're told R. Kelly's camp thinks the note shows her parents were generally willing to let Azriel travel with him.

He might end up having to make that point in court. Stay tuned.