Demi Lovato's representative has brushed off rumors suggesting the pop star is engaged to her on/off boyfriend Wilmer Valderrama.

The Skyscraper hitmaker was said to have reunited with the former That '70s Show star in September (13), and she sparked speculation he had recently proposed after sharing a photo of herself sporting a gold band on her ring finger online on Thursday (30Jan14).

She later took to her Twitter.com page to gush about Valderrama in a sweet note to celebrate his birthday, writing, "To the funniest, most inspiring, incredible man I know.. @WValderrama - thank you for helping me get through my darkest moments.

"You truly are a light to this world and to my life. Happy Birthday!!!"

The birthday boy, who turned 34 on Thursday, replied to the message by tweeting, "who needs presents when the gift of you is real to us all.. Thank you for the blessing of knowing you."

However, a spokesperson for Lovato has denied the marriage gossip, telling GossipCop.com the new jewelry is "not an engagement ring".

The couple was previously reported to have dated for several months in 2011, but the stars have never confirmed their romance.

Last year (13), Lovato said, "I share so much with the world that I like to keep some things private and it's something I don't want to talk about. You can do whatever you want. You can be seen holding hands. You can be seen wherever, but it doesn't mean you have to talk about it or acknowledge it because it opens up a can of worms."

Lovato previously dated Joe Jonas, while Valderrama has also been linked to Lindsay Lohan and Minka Kelly in recent years.