Social media star Boo the Pomeranian dog has died at the age of 12.

His owners say he had shown signs of heart problems ever since his "best friend" - a dog called Buddy - died in 2017.

"We think his heart literally broke when Buddy left us," they wrote on his official Facebook page.

Boo had 16 million followers on Facebook, made TV appearances, and even released a book called Boo - the life of the world's cutest dog.

Boo and his friend Buddy had lived together for 11 years - before Buddy passed away in September 2017 at the age of 14.

His owners, who are from the US, say Boo passed away in his sleep early Saturday morning and they were "heartbroken".

They added: "But we find comfort knowing that he is no longer in any pain or discomfort.

"We know that Buddy was the first to greet him on the other side of that rainbow bridge, and this is likely the most excited either of them have been in a long time."

View this post on Instagram

#tbt to when human was too lazy to wash our paws before our holiday card photo.

A post shared by Buddy Boo Blue Benny (@buddyboowaggytails) on Dec 6, 2018 at 3:18pm PST

His owners went on: "Boo, we love you with all our hearts and will miss you until the day we meet again.

"Have fun running around with Buddy and creating adorable mischief wherever you guys go."

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perks of being a pup: clothes optional; sunnies sufficient. [photo credit: @thegracechon]

A post shared by Buddy Boo Blue Benny (@buddyboowaggytails) on Jul 16, 2015 at 12:20pm PDT

As well as releasing a book, Boo met with celebrities such as Seth Rogan, became an ambassador for a US airline and appeared on several TV shows in the US.

His owners said they'd received many notes over the years from people sharing their stories of how Boo "brightened their days and helped bring a little light to their lives during difficult times".

"And that was really the purpose of all this...Boo brought joy to people all over the world."