Demi Lovato's on/off boyfriend has gushed about her on Twitter sparking rumours they have reconciled.

Former 'That 70s Show' star Wilmer Valderrama shared his feelings for the 'Heart Attack' singer in a touching message for her 21st birthday on Tuesday (20.08.13), which she is celebrating in Africa.

The 33-year-old actor wrote: ''I've been so proud of the strength & commitment you've gained 4 life, keep inspiring us all! Happy birthday my @ddlovato! With & by u always.''

Demi and Wilmer started dating in 2010 shortly after her split from Joe Jonas.

They split up after a few months but were linked again in 2011 before they split again.

But the duo were then spotted hanging out together again in February and shared a romantic Valentine's Day dinner together in Los Angeles.

An eye-witness said at the time: ''They ordered almost everything on the menu, but they didn't touch it for at least an hour. They stared into each other's eyes the whole time.''

The pair were also spotted ''making out'' at the grand opening of Topshop in L.A.

Demi - who checked into rehab for three months in October 2010 where she successfully received treatment for her eating disorder and problem with self-harming - revealed earlier this year that she is finally in a place where she wants to be in a relationship.

The 'X Factor' USA judge said: ''I'm in a place where I can finally trust and love and be a better person for the man I am in a relationship with, instead of being a clingy girlfriend or getting jealous.''

The singer is celebrating her milestone birthday in Africa as she is working there with a charity.

She tweeted: ''Thank you all for the birthday wishes!!! Having the best birthday ever in Africa with @freethechildren!!!''