Justin Bieber has been accused of fathering a fan's child following claims that she had sex with the underage pop star. But Bieber's representatives have vigorously denied the claims.

Twenty-year-old Mariah Yeater claims her son Tristyn was born after a 30-second backstage romp with the pop singer following a concert at the Staples Centre in Los Angeles in October last year.

Yeater listed herself as the child's mother on the birth certificate but failed to complete details of the father on the form.

Four months on, she has claimed the child belongs to Bieber, 17, and filed a lawsuit on Oct. 31 demanding a DNA test and financial support.

Yeater is insisting the Canadian-born star should cough up £7,500 per month for four-month-old Tristyn.

Bieber's representatives have denied Yeater's claims, describing them as "malicious, defamatory and demonstrably false".

"We will vigorously pursue all available legal remedies to defend and protect Justin against these allegations," they told E! Online.

The claims could also carry legal implications for Yeater. Although she would have been 19-years-old at the time of the alleged intercourse, Justin would have only been 16 - under Californian law, that would mean she would be guilty of statutory rape of a minor.

However, Yeater's lawyers Lance Rogers and Matthew Pare said they have "credible" evidence and will fight until Bieber agrees to a DNA test.

"Mariah Yeater is pursuing a modest and rightful claim. There is credible evidence that Justin Bieber is the father of the baby. The evidence will be revealed in court," they said in a statement.

Yeater's lawyers insisted she was not seeking "exorbitant compensation".

"She merely request he provide a DNA sample to verify he is the father, and, if so, help provide child support," they said.

Yeater's lawyers have encouraged Bieber to settle the case before the potential court hearing takes place next month.

But some of Bieber's fans, who call themselves Beliebers, vented their anger towards Yeater and issued death threats via Twitter. One Belieber tweeted: "I could kill this b----, really she messes with my baby!!! SHE IS SOOO DEAD."

Another tweeted a poem: "Dear Mariah Yeater, Roses are red, violets are blue, stay away from Bieber or Beliebers will kill you."

Bieber, currently dating actress Selena Gomez, 19, and a devout Christian, took to Twitter and said "forget the hype... all the rumors" and to reassure his fans that he is "gonna focus on the positives... the music."