BANGKOK - Myanmar must address anti-Muslim propaganda and stamp out a culture of impunity for religious violence or risk "catastrophic" levels of conflict, a rights group warned Tuesday.

Physicians for Human Rights described attacks on Muslims, that have swept the country since fighting first broke out last year as "widespread and systematic", in a report examining unrest that has killed around 250 people and left tens of thousands homeless.

The US-based group said that while the situation in the country currently appeared calm, a failure to properly investigate and deal with the causes of the tensions risks further clashes.

PHR reported that "the brazen nature of these crimes and the widespread culture of impunity in which these massacres occur form deeply troubling preconditions that make such crimes very likely to continue". "If these conditions go unaddressed, Burma may very well face countrywide violence on a catastrophic level, including potential crimes against humanity and/ or genocide," it continued, using the country's former name.

Myanmar has strongly denied previous accusations by watchdog Human Rights Watch of ethnic cleansing against the Rohingya in Rakhine state. Attacks against Muslims, who are thought to make up at least four percent of Myanmar's population, have thrown the Buddhist-majority nation's much-hailed emergence from military dictatorship into question.

Meanwhile, dozens of Muslim Rohingya boat people from Myanmar escaped Tuesday from a Thai immigration centre, police said, as a rights group called for all of them to be freed from detention. Some 87 Rohingya men broke out of the detention centre housing 137 asylum-seekers in Sadao, a southern district bordering Malaysia.

They were among nearly 2,000 Rohingya who have been held - some for many months -for entering Thailand illegally. Local media reports said they punched a hole in the ceiling and climbed out using a rope made from their clothes.

Authorities are searching for the runaways and have rearrested two of them, according to Suwit Choensiri, a police commander in Songkhla province which includes Sadao.

"The reason for their escape is the pressure they are under after they being detained for long time with no idea of their future," he added.

Many of the asylum-seekers have been locked up in the overcrowded and reportedly insanitary centre for several months.

Thousands of Muslim Rohingya boat people - including women and children - have fled Myanmar since Buddhist-Muslim clashes a year ago in its western state of Rakhine.

Those who arrived in Thailand have been "helped on" by the kingdom's navy towards mainly Muslim Malaysia - their destination of choice - or detained as illegal immigrants.

Thailand initially said the asylum-seekers would be allowed to stay for six months while the government worked with the UN refugee agency UNHCR to try to find other countries willing to accept them.

But overseas help has not been forthcoming so far, leaving the refugees in limbo and separated from their families.