Taylor Swift has revealed that her friendship with Selena Gomez is so strong that she almost sees her as a sister.

The two US pop starlets have become close friends over the years and it seems that bond is still running strong.

Writing on Twitter yesterday (July 22), Swift paid tribute to Gomez as she celebrated a landmark birthday. The 'Love Story' lady said:

"Happy 21st birthday to my friend, and the closest thing I've ever had to a sister, @selenagomez!!"

She also uploaded a photo of the pair striking a pose together next to a mermaid ornament.

As previously reported, Swift recently revealed that she is in full-on songwriting mode as she sets her sights on following up her 2012 record 'Red'. She said:

"The floodgates just opened the last couple weeks. I'm getting to that point where I'm irritating to be around because I'll be with you for half the conversation and then the second half of the conversation I'm clearly editing the second verse of whatever I'm writing in my head." Watch Taylor Swift perform live below: