The pop star, 26, had earlier issued a statement on Twitter to deny the alleged incidents in 2014 and 2015.

Bieber has now sued both users for defamation in a $US20 million ($A29 million) lawsuit filed in Los Angeles.

He accused the users of "outrageous lies" that are "provably fabricated".

The lawsuit adds: "The accusations are factually impossible, and disproven both by indisputable documentary evidence and the individuals' own admissions."

Bieber's legal team says the "malicious statements" may be from the same person using different accounts, and the allegations are "absolutely false, and their respective heinous accusations that Bieber engaged in the alleged sexual assaults are outrageous, fabricated lies".

The first allegation against the singer came from a social media user claiming to have been assaulted at a hotel in Austin, Texas, on March 9 2014.

She claimed that following a surprise performance at a bar, Bieber invited her and two friends to a Four Seasons hotel, where she alleges she was assaulted.

In response, Bieber posted screenshots from news stories from the day of the alleged attack, which show him with his then-girlfriend Selena Gomez.

He did not stay at the Four Seasons hotel on that date, Bieber says.

The second allegation alleges Bieber attacked a woman at the Langham Hotel in New York in May 2015.

Bieber's lawsuit describes the allegation as an "elaborate hoax" that is motivated by the accuser's "desire for fame and attention".

In his initial response to the claims, Bieber, who is married to the model Hailey Bieber, said: "Rumours are rumours but sexual abuse is something I don't take lightly.

"I wanted to speak out right away but out of respect to so many victims who deal with these issues daily I wanted to make sure I gathered the facts before I made any statement."

Australian Associated Press