The UN on Tuesday questioned the credibility of a government-led investigation into allegations of abuses against the Muslim minority Rohingya community in Myanmar.

"The existing Commission is not a credible option to undertake the new investigation," UN Special Adviser on the Prevention of Genocide Adama Dieng said.

His comments follow an announcement by the Myanmar government to launch a fresh investigation into the allegations after the High Commissioner of Human Rights in a report last week refuted the clean chit given to the army by a government committee, headed by Vice president Myint Swe.

The report had blamed the army, deployed in the state of Rakhine after an alleged armed assault by Rohingya rebels in October, of carrying out ethnic cleansing in the region.

"I urge that any investigation be conducted by a truly independent and impartial body that includes international observers," said Dieng, adding that the differences between the reports published by the government body and the UN were disturbing.

The army offensive has, meanwhile, blocked access to humanitarian aid – on which thousands of people in the region depend – and has also barred observers and independent media from accessing the northern part of the state.

The Human Rights Watch also reported fresh violence against the Rohingyas such as rape of women, including minors, by soldiers.

"These horrific attacks on Rohingya women and girls by security forces add a new and brutal chapter to the Burmese military's long and sickening history of sexual violence against women," said HRW's Senior Emergencies Researcher Priyanka Motaparthy.

Motaparthy also denounced the Myanmar government for its lopsided investigation, and demanded an international and independent probe.

"The government should stop contesting these rape allegations and instead provide survivors with access to necessary support, health care, and other services," Motaparthy added.

More than a million Rohingyas live in Rakhine, where they face increasing discrimination from the country's authorities, who do not recognise them as Burmese citizens.