It seems things between Justin Bieber and Selena Gomez are officially heating up, with the pair cementing their reunion with a bowling date on Sunday night (September 14).

According to E! News, the cute couple were seen kissing at JB's pal, Ryan Butler's birthday party at Lucky Strike in Hollywood.

A source told the website: "Justin rented out Lucky Strike's Luxe room for the celebration.

"Additionally, he was seen kissing and flirting with his on-and-off again girlfriend, Selena Gomez."

The happiest I've seen him in a while.. #goodcompany #heloveshisfriends #happy20thbabepic.twitter.com/pt9kJMD3og

— Ashley (@AshGuin) September 16, 2014

[related]A snap of the pair has since emerged on Twitter, courtesy of Butler's girlfriend Ashley Guin, which shows Sel-Go looking lovingly at Justin, who has a huge smile on his face.

She caption the pic - which also shows Ryan sitting alongside the couple: "The happiest I've seen him in a while.. #goodcompany #heloveshisfriends #happy20thbabe."

The news comes after Justin and Selena were spotted in Las Vegas together, with the Boyfriend singer taking Gomez to watch his boxer friend Floyd Mayweather fight Marcos Maidana.