By Hilary Russ

NEW YORK, Aug 27 (Reuters) - As football fans get ready for the start of the National Football League season on Sept. 5, they may think the league will finally endorse a legal U.S. sports betting operator.

They would be wrong.

"We haven't opened up the category" in the United States, Christopher Halpin, the NFL's Chief Strategy & Growth Officer, told Reuters on Tuesday.

Although the NFL has an official casino partner with Caesars Entertainment Corp and an official data provider to sportsbooks with the Swiss company Sportradar AG, it has no official sports betting partner as all the other major leagues do.

Halpin said the league may experiment with sportsbook partnerships in a couple international markets before making any moves to do the same in the United States.

"We're being thoughtful and continuing to expand," he said on the sidelines of a media event to launch the NFL's 100th season.

Though the NFL is the most bet-upon professional sports league, its die-hard fans are not the ones betting on the games. Only about one-fifth of NFL fans place wagers, Halpin said.

The league does not want to "alienate a lot of fans needlessly" by rushing into sports wagering too quickly, he said.

Professional U.S. basketball, baseball, hockey and soccer leagues have all named various sportsbook operators as official partners, including Caesars, MGM Resorts International, Flutter Entertainment's FanDuel, DraftKings and William Hill.

The partnerships, in which a league and sportsbook operator usually agree to share trademarks and logos as well as betting data to monitor integrity, have followed a U.S. Supreme Court ruling in May 2018.

That decision overturned a 1992 federal ban on sports betting in most states outside of Nevada, opening the doors to other states that wanted to legalize, regulate and tax the activity.

Currently twelve states are offering live sports betting, with more expected to come online in coming months. (Reporting by Hilary Russ Editing by Sonya Hepinstall)