The UN agency tasked with facilitating Myanmar´s first census in decades on Tuesday said it was "deeply concerned" over a government decision to bar Muslims from identifying 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 (UNFPA) said in a statement.

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

The move followed fresh unrest in western Rakhine State where Buddhist nationalists have campaigned vociferously 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.

"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 Tuesday census takers were not collecting any data on people identifying as Rohingya -- the vast majority of the displaced.

An AFP journalist said teams flanked by police and soldiers would ask for a person´s ethnicity and would walk away if the householder replied "Rohingya".

The census is the first since 1983 for the country following years of junta rule, and is meant to fill gaping deficits in information that mean even the country´s population is a broad estimate.

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

UNFPA 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 Britain.

A further $5 million was provided by UNFPA 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.

UN Secretary General Ban Ki-moon called the Myanmar president Thein Sein on Sunday to call for protection of civilians and aid staff. He also noted the tensions raised by the census and called for it to be conducted in a "credible" manner.