LONG-SIMMERING ethnic tensions in Burma exploded in January. In the village of Du Char Yar Tan in Rakhine state, at least 48 people were killed in two separate incidents when Buddhist mobs went on a rampage against Rohingya Muslims. A police sergeant was reported to have been captured and killed by the Muslim villagers.

The bloodshed was bad enough for the Rohingya, a small and persecuted minority in a land that is predominantly Buddhist. But events since the attacks have taken a discouraging turn at a time when Burma, also known as Myanmar, appeared to be making strides toward openness, democracy and modernization.

The government refused to acknowledge the violence, although the U.N. high commissioner for human rights cited what she called “credible” information that more than 40 Rohingya were killed and urged for a full, prompt and impartial investigation. That credible evidence reportedly included the discovery of the severed heads of at least 10 Rohingya bobbing in a water tank, according to the New York Times.

Doctors Without Borders, which had an extensive program in Burma, treated at least 22 victims of the violence in Du Char Yar Tan. In response, Burma issued a written order to Doctors Without Borders in February to cease all operations in the country. Before the order, the organization had provided medical services to the approximately 700,000 people in the state, including 200,000 living in camps and isolated villages. The group had been present in Rakhine state for two decades, but now more than 100 medical staff have left and all clinics are closed, leaving untold thousands without desperately needed medical care. Burma has permitted operations to resume elsewhere in the country.

In recent days, Burma has begun taking its first national census since 1983, which could be important in planning for the nation’s huge needs in health, education and development. But the census has further deepened tensions. Last Wednesday and Thursday, mobs attacked homes and offices of international aid workers in Sittwe, the capital of Rakhine state, and some workers were evacuated. The riots were triggered by an apparent misunderstanding over the removal of a Buddhist flag from the warehouse of a German nongovernmental organization during the census.

Burma has earned applause for its tentative opening toward democracy. But the progress is threatened by the deep ethnic divisions and the violence. The flight of aid organizations will cause already needy people to suffer more. Before any more damage is done, Burma’s leadership needs to forthrightly come to grips with the volatile situation and conduct credible investigations into the violence. It would be refreshing to hear Aung San Suu Kyi, the Nobel peace laureate and opposition leader, find her voice and speak out for tolerance. Burma should also allow Doctors Without Borders to return, and it should finish the census in a manner that makes it clear the exercise will benefit all its people and is not an excuse to vent ethnic hatreds.