Joe Jonas and Taylor Swift: New Couple?

says that Joe Jonas is just a friend. But the Jonas Brothers cutie fueled all those "Are they an item?" rumors when he turned up at the country star's concert on Sunday night.

Hiding among a group of people, Jonas, 19, arrived at the Rascal Flatts concert in the West Palm Beach Cruzan Amphitheatre five minutes before Swift, 18, took the stage.

"I spotted Joe quickly, but he was trying to hide from the crowds by tucking in-between a few people," says Mishelle Rivera, Assistant Promotions Director for Miami's Y-100 Radio. "He was wearing a baseball cap and real casual dress so he wouldn't stand out."

Rivera and her friends saw Jonas near the soundboard - next to Swift's dad - ready to watch Taylor, who sparkled in a sleeveless turquoise sequin dress and cowboy boots, perform her number one hit, "I'm Only Me When I'm with You."

"He watched only her performance, then walked backstage to the meet and greet area," says Rivera. "But when he saw people start to recognize him, he hid. It was real obvious he didn't want people to know he was there to see Taylor."

A surprise guest at a couple recent Jonas Brothers concerts, Swift shot a cameo for the band's upcoming 3-D movie in New York City last week.

In July, she denied to PEOPLE that she and Jonas are an item, saying of Joe and his two brothers, "We're all friends."

Reached by PEOPLE on Thursday, Swift said through her rep: "He's an amazing guy and anybody would be lucky to be dating him."

And on Aug. 5, when Ryan Seacrest asked Jonas if he was dating Swift, he replied: "She's a great girl ... I think anybody would love to go on a date with her."

Asked whether the two are dating, Swift's publicist said, "They are friends and she's friends with all the jonas brothers." A rep for the jonas brothers had no comment.

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