Sofia Richie and Kylie Jenner. Picture: Instagram

Sofia Richie has branded Kylie Jenner her "best friend".

The 21-year-old model - who is dating Kourtney Kardashian's ex Scott Disick - dubbed the 'Keeping Up With The Kardashians' star her closest pal as they celebrated Kylie's birthday in Capri, Italy.

In a video of Kylie's 22nd birthday celebrations posted to her social media account, Sofia tells the camera: "Kylie, you're 22 and you've always been one year cooler than me. But I love you. You're my best friend. I don't know what life would be like without you."

It comes after a source revealed Sofia helped Kylie cope after her fall out with her ex best friend Jordyn Woods.

One insider said: "Sofia really helped Kylie through the emotional roller coaster during the Jordyn situation. She was really there for her and took her side through everything. 

The situation with Jordyn made them closer. Kylie also trusts Sofia around Stormi, which is very important to her. Sofia has been around since Stormi was born and Kylie feels comfortable with having her around her child after getting to know her better. They have now formed an irreplaceable bond."

Now, Kylie and Sofia hang out whenever they can, and the insider insists they love having "girls' nights" at home or on the town.

They added: "Kylie loves how fun Sofia is and has been inviting her over to her house more often to hang just the two of them. They love staying home and watching movies and cooking together while hanging with Stormi. Kylie knows whenever she wants a girls' night out on the town, she can always count on Sofia for a good time."