Myanmar security forces opened fire on Rohingya Muslims, committed rape and stood by as rival mobs attacked each other during a recent wave of sectarian violence, a rights watchdog said Wednesday.

The authorities failed to protect both Muslims and Buddhists and then "unleashed a campaign of violence and mass roundups against the Rohingya," New York-based Human Rights Watch said in a report.

The violence which erupted in June in western Rakhine state between ethnic Buddhist Rakhine and Rohingya has left about 80 people dead, based on official figures, and forced tens of thousands to flee as homes were torched.

"What is remarkable is that if the atrocities that we saw in Arakan (Rakhine) had happened before the government reform process had started, the international reaction would have been swift and strong," said HRW Asia deputy director Phil Robertson.

"But the international community appears to be blinded by a romantic narrative of sweeping change in Burma (Myanmar), signing new trade deals and lifting sanctions even while the abuses continue," he told a news conference.

The report, based on dozens of witness interviews, said that the events in Rakhine state "demonstrate that state-sponsored persecution and discrimination persist" despite the government's pledge to end ethnic unrest.

Police and paramilitary forces "opened fire on Rohingya with live ammunition," it added.

It quoted one Rohingya man in the Rakhine state capital Sittwe as saying that security forces watched as a Buddhist mob started torching houses.

"When the people tried to put out the fires, the paramilitary shot at us. And the group beat people with big sticks."

Decades of discrimination have left the Rohingya stateless, and they are viewed by the United Nations as one of the world's most persecuted minorities.

Speaking a Bengali dialect similar to one in neighbouring Bangladesh, the Rohingya are seen as illegal immigrants by the Myanmar government and many Burmese, and many have attempted to flee overseas in rickety boats.

The violence, along with fighting in northern Kachin state, has cast a shadow over widely praised reforms by Myanmar's President Thein Sein over the past year, including the release of hundreds of political prisoners and the election of opposition leader Aung San Suu Kyi to parliament.