Bieber, Selena's surprise duet causes Twitter freak-out

Justin Bieber and Selena Gomez's surprise collaboration for unofficial duet 'Strong' has caused Twitter freak out.

The title, which was aired on Apple Music's OVO Sound Radio, features the 21-year-old singer singing the lyrics "What should I do? Should I go?/Who do I talk to? Nobody knows," while the 'Good For You' hit-maker croons, "Wishing I was better at this d**n thing/Cause it's just been too long and I can't change," Us magazine reports.

The fans have flooded the social media site with their reactions.

One fan wrote, "JUSTIN AND SELENA MADE A SONG AMD I'M NOT OKAY," while another wrote, "this @selenagomez and @justinbieber song is a lot for me to deal with."

Another tweeted, "wait selena and justin did a song together."

According to sources, it is unclear when was the song recorded or whether it will appear in Bieber's new album 'Purpose.'

However, the singers, who dated on-and-off for three years until 2013, have yet to comment on the track.