A top aide to United Nations Secretary-General Ban Ki-moon has arrived in western Burma, where security forces are presiding over an uneasy calm following days of deadly sectarian clashes between Buddhists and Muslims.

U.N. officials told VOA that Vijay Nambiar, the international body's top envoy for Burma, arrived Wednesday in western Rakhine state, where government officials say more than 20 people have been killed.

President Thein Sein has declared a state of emergency and sent army troops into Rakhine, which has seen a wave of rioting and arson that has destroyed hundreds of homes.

Violence erupted June 3 when a mob of Rakhine Buddhists in Rakhine ambushed a bus and killed 10 Rohingya Muslim passengers, in apparent retaliation for the earlier rape and murder of a Buddhist woman by three Muslims.

Activists reported that the situation calmed on Wednesday with the presence of the army and dusk-to-dawn curfews imposed on many parts of the region.

But Chris Lewa of the Arakan Project, which monitors western Burma, tells VOA there is now concern that Burmese security forces may be making arbitrary arrests, particularly in the border city of Maungdaw.

“In Maungdaw, the situation has calmed down a little bit last night with the presence of the army. But there are still a number of incidents that have taken place today in different parts of the township, especially around the city. But we see now it's mostly people being arrested. And nobody knows what is going to happen to them or for what reason they have been arrested.”

Burma's army has a record of human rights abuses against ethnic minority groups. And there are reports that riot police in the region are favoring the Rakhine over the minority Rohingya.

Lewa says, in the short-term, the army's presence seems to be reducing the violence. But she also said the Burmese government must work to repeal laws that deny citizenship for Rohingya Muslims in order to ensure national reconciliation.

The unrest has highlighted longstanding tensions between Buddhists and minority Rohingya Muslims. Burma does not classify its estimated 800,000 Rohingyas as Burmese citizens, instead regarding them as illegal immigrants from neighboring Bangladesh.

In predominately Muslim Bangladesh, officials say their border guards have turned back more than 500 Rohingya Muslims trying to flee the fighting. Bangladesh's Foreign Ministry says it is not in the country's best interest to allow the Rohingyas in.

New York-based Human Rights Watch released a statement Wednesday calling on Bangladesh to immediately open its borders to Rohingyas seeking sanctuary, saying failure to do so is “putting lives at grave risk.”

President Thein Sein has warned the violence could jeopardize Burma's nascent reform process. He said the unrest is fueled by “hatred and revenge based on religion and nationality” and noted it could spread to other parts of the country. If that happens, he said the country's stability, peace, and democratization process could be severely affected.