Tasunba Nourin, a doctor with the International Organisation for Migration (IOM) told Reuters injuries include bite marks, vaginal tearing and "signs that a firearm was used to penetrate women".

Nourin Tasnupa, who works at a UN-run clinic in a refugee camp near the Bangladesh border called Leda told Agence France Press (AFP) that most of the survivors she had treated were beaten and raped and had bruising on their bodies and bite marks on their breasts and genitals.

"People don't want to share these incidents even with family members," she said.

A social stigma surrounds rape in the Rohingya's conservative Muslim society.

AFP quoted one victim gripping the hand of her six year-old son with tears in her eyes saying three soldiers raped her.

"When they left I ran out of the house with two of my children and followed the crowd of people running for their lives," she said.

The woman said she was bleeding for three days before crossing the border in Bangladesh.

Pramila Patten, the UN's Special Representative on Sexual Violence, said she is "gravely concerned" about security operations in Rakhine that the UN has described as "textbook ethnic cleansing".

She said survivors have described sexual violence being used as a "calculated tool of terror to force targeted populations to flee".

Almost all the victims told doctors the perpetrators were men in uniform who they identified as Myanmar soldiers.

One woman said she was raped by five soldiers while others looked on.

More than 430,000 Rohingya have fled Rakhine for Bangladesh since late August when Myanmar security forces implemented a sweeping crackdown after insurgent attacks on 30 police posts and a military base.

The exodus is the largest movement of civilians in Asia since the 1970s and has created what UN and aid agencies say is an emerging humanitarian catastrophe in Bangladesh mega camps.

Myanmar officials have mostly dismissed allegations of sex abuse by soldiers as militant propaganda.

Zaw Htay, spokesman for Myanmar's de factor leader Aung San Suu Kyi, said authorities would investigate any allegations brought to them.

Ms Suu Kyi, a Nobel laureate widely condemned for failing to speak-up for more than one million Rohingya, promised last week that any perpetrators on abuse in Rakhine would face the full force of the law.

But her government has refused to allow a UN fact-finding mission to go to Rakhine that was set up to investigate what a UN report in February said amounted to a "calculated policy of terror" under the guise of a military lockdown of Rohingya villages months earlier.

The report described how soldiers raped women, stomped on the stomach of a woman in labour and slit the throat of an eight-month-old baby when he started crying while his mother was being raped.

UN investigators have arrived in the Bangladesh camps to investigate the atrocities, including sexual attacks.

A report of the UN Secretary General in April said sexual assaults were "apparently employed systematically to humiliate and terrorise" in the Rohingya community.

Before she was swept into office at elections in 2015 Ms Suu Kyi had spoken of rape being used as a tool of division in her country's myriad ethnic conflicts.

Reuters cites a situation report from aid agencies in Bangladesh referring to 350 people who had been treated for "life-saving care" relating to gender-based violence since late August - a broad term that includes rape, attempted rape and molestation.

Kate White, emergency coordinator for Medecins San Frontiers (Doctors without Borders) said the charity had treated at least 23 cases of sexual and gender-based violence.

"This is a fraction of the cases likely to be out there," she said.

Meanwhile, survivors in the Bangladesh camps say tens of thousands of Rohingya are huddling near beaches and forests waiting to escape Rakhine.

Bangladesh has complained to the UN that Myanmar soldiers have been laying internationally banned anti-personnel landmines across the border.

Myanmar's military denies the claim.

Rohingya have been denied citizenship and other basic rights in Buddhist-majority Mynamar despite that they have lived there for generations.

-With agencies