The United Nations High Commissioner for Human Rights (UNHCR) has called on authorities in Australia and Nauru to do more to help a pregnant refugee who says she was raped.

The 23-year-old, known as Abyan to protect her identity, was transferred to Australia for an abortion after claiming she was sexually assaulted.

She was subsequently returned to Nauru without undergoing the procedure.

Rupert Colville, a spokesman from the Office of the High Commissioner for Human Rights (OHCHR), said the woman required further assistance.

"We call upon Australia and Nauru to urgently provide a decent option for Abyan... to obtain adequate mental and physical care and to terminate her pregnancy if she desires," Mr Colville said in a statement.

"OHCHR has been in direct contact with her.

"Abyan is in a very fragile mental and physical condition and is deeply traumatised by her experiences since the day of the alleged rape."

He said the UNHRC was concerned at reports police on Nauru had failed to take action against alleged perpetrators of violence against women, and that Abyan was fearful for her safety.

"She has refused to give information to the Nauru police about her attacker because she is understandably afraid of reprisals," Mr Colville said.

"She does not feel safe, given that her alleged attacker lives on Nauru, which is a very small island state with a population of around 10,000."

The UNHCR said it was "very disturbed" at the growing number of sexual assault and rape allegations being made since Australia re-established its offshore immigration detention network in 2013.

There have been conflicting reports about what happened when Abyan was sent to Australia for treatment.

The Federal Government said she chose not to undergo the abortion, but lawyers acting for Abyan said she was not given adequate support on her arrival to Australia and wanted more time to make a decision.