The report, which includes interviews with more than 100 people on both sides, and visits to all major camps where displaced Muslims are now living, concludes that there is compelling evidence of official involvement in the violence in Arakan state in October. More than 180 people were killed and 100,000 left homeless by anti-Rohingya violence last year.
The report states: "The absence of accountability against those to blame lends credence to allegations that this was a government-appointed campaign of ethnic cleansing in which crimes against humanity were committed."
This week the European Union is due to eliminate the trade and economic sanctions on Burma – except the arms embargo – which were suspended one year ago, in recognition of the country's moves towards democracy. But in light of the report's findings, David Mepham, of HRW in London, told The Independent: "Lifting all the sanctions on Burma is premature and unjustified. European governments are relinquishing their leverage over Burma when concerted pressure is most needed to investigate anti-Muslim violence and crimes against humanity."
The spark for the violence which began in the far west of Burma last June was the rape and murder of a 28-year-old Arakanese Buddhist woman by three Muslims. Tensions between Buddhists and Muslims that have simmered and periodically erupted for generations quickly exploded into an orgy of violence in which lives and homes on both sides were destroyed.
But when the violence flared up again in October 2012, it was in the form of simultaneous attacks on Muslim communities in nine of Arakan's 21 townships.
"On 22 October, after months of meetings and public statements promoting ethnic cleansing, Arakanese mobs attacked Muslim communities in nine townships, razing villages and killing residents while security forces stood aside or assisted the assailants. Some of the dead were buried in mass graves, further impeding accountability," says the report, entitled "All You Can Do Is Pray".
In July, President Thein Sein, the former general who was building a reputation as a democratically minded reformer, had appeared to approve a plan to expel Rohingyas from Burma. "We will take care of our own nationalities," he said. "But Rohingyas who came to Burma illegally are not of our nationalities and we cannot accept them here… They can be settled in refugee camps… If there are countries that would accept them, they could be sent there."
That proposal was echoed in crude pamphlets distributed in the state. One of them was baldly headed: "Arakan Ethnic Cleansing Program of bad pagan Bengalis… taking advantage of our kindness to them".
HRW documents other attempts during the summer of 2012 to inflame the fear and hatred of the majority community against the Muslim minority, while security forces demolished mosques and homes abandoned during the June violence, making it difficult for the Muslims to return the areas where they had previously lived.
Then on 22 October came the co-ordinated assault. "Carrying machetes, swords, spears, home-made guns, Molotov cocktails and other weapons, sizeable groups of Arakanese men simultaneously descended on Muslim villages in several townships in a coordinated fashion," the report says. "Far from being a brief flash of violence, the carnage lasted over a week in nine of the state's 17 townships… Most of these areas had not experienced violence in June."
In perhaps the worst case, in Yan Thei village in Mrauk-u, Arakan's historic capital, the day-long massacre led to the deaths of 70 Rohingyas, of whom 28 were children, 13 of them under the age of five.
Today, with the rainy season about to descend, around 120,000 Rohingyas are living in hastily improvised camps around the state, barred from returning to their homes or going anywhere else. Many of the most desperate have fled: the United Nations refugee agency estimates that some 13,000 people, including Rohingyas and Bangladeshi nationals, took to the Bay of Bengal in flimsy boats during 2012.
After October's murderous attacks, President Thein Sein softened his rhetoric. In a letter to the UN Secretary-General in November, he promised that "once emotions subside on all sides" his government was prepared to "address contentious political dimensions ranging from resettling of displaced populations to granting of citizenship". Critics voiced suspicion that this more emollient tone was aimed at the ears of US President Barack Obama on the eve of his historic visit to Burma in November, which were heightened when the Foreign Ministry released a statement the following month referring to "so-called Rohingyas" and "Bengalis". It denied that the government had played any part in the attacks on them.