Pharrell Williams' monolithic track 'Happy' has spent another week at No.1 in the US.

The 'G I R L' star has enjoyed worldwide success with the song and it has continued its fine run of form on the Billboard Hot 100.

It has now spent four weeks at the chart's summit, with Katy Perry's 'Dark Horse' at No.2.

Currently at No.3 on the list is John Legend's 'All of Me', whilst No.4 is Jason Derulo's 'Talk Dirty'.

Completing the top five of this week's Hot 100 is Bastille's 'Pompeii', which is followed by Lorde and her track 'Team' at No.6.

Pharrell meanwhile is helping the United Nations celebrate their International Day of Happiness today (March 20) by acting as their celebrity ambassador.

Watch the video for Pharrell's 'Happy' below: