The UN agency that helped Myanmar prepare for its first census in decades said on Tuesday it was deeply concerned about a government decision to bar Muslims from identifying themselves as Rohingya, warning it could stoke tensions.

Myanmar had said it would conduct the rare survey in accordance with international standards and "explicitly agreed" to allow people to be able to choose their own ethnicity, the United Nations Population Fund said in a statement.

The agreement was dropped on the eve of the census, which began on Sunday and runs until April 10, when Myanmar announced that "Rohingya" would not be accepted by census takers.

The move followed fresh unrest in the western state of Rakhine where Buddhist nationalists have campaigned vehemently against the inclusion of the term, fearing it could herald political rights for the stateless Muslim Rohingya.

"UNFPA is deeply concerned about this departure from international census standards, human rights principles and agreed procedures," the agency said.

"We are concerned that this could heighten tensions in Rakhine state, which has a history of communal violence, as well as undermining the credibility of census data collected."

Myanmar, a predominantly Buddhist nation of about 60 million, held its last count in 1983 and experts say the information being gathered is crucial for national development and planning.

In the sprawling camps near the Rakhine capital of Sittwe, where 140,000 people made homeless by sectarian fighting two years ago live, there were signs on Tuesday that census takers were not collecting any data on people identifying themselves as Rohingya - the vast majority of those displaced.

In village after village in Rakhine, enumerators were asking households to identify their ethnicity. When the answer was "Rohingya", they said "thank you", turned around and walked away.

The census follows years of junta rule and is aimed at filling huge gaps in information, with even the figure for the country's population just a broad estimate.

But the count has come under fire from critics who say organizers have failed to take sufficient account of the country's volatile ethnic and religious divides.

The UN fund has provided a wealth of technical assistance in the 18-month preparation period, including designing the questionnaire, data processing and country mapping.

The exercise has received some $45 million from nine countries, including Australia, Germany, Norway and the United Kingdom. A further $5 million was provided by the fund and $15 million by the government.

The British embassy in Yangon on Sunday expressed concern that census respondents might not be allowed to register their ethnicity freely.

Foreign aid workers fled Rakhine last week after Buddhist mobs attacked their offices as tensions escalated in the run-up to the census.

An 11-year-old girl was killed by a stray bullet after police fired warning shots to disperse angry crowds in Sittwe.

AP-AFP