More than 20 people have been killed in western Burma's Rakhine State, as international pressure mounts for an end to sectarian fighting between ethnic-Rakhine Buddhists and Rohingya Muslims.

President Thein Sein has declared a state of emergency and sent army troops in Rakhine, which has been hit with a wave of rioting and arson in recent days. Hundreds of homes have been destroyed.

U.S.-based Human Rights Watch said the violence is “spiraling out of control.” A Human Rights Watch official said there are reports riot police in the region are favoring the Rakhine over the Rohingya.

There was a heavy security presence Tuesday in the regional capital, Sittwe, where fires dotted the area and people ran to escape the chaos.

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 the country's best interest to allow the Rohingyas in.

The violence erupted a week ago when a Buddhist mob in Sittwe ambushed a bus and killed 10 Rohingya passengers, mistakenly believing they were responsible for the recent gang-rape and murder of a Buddhist woman.

The unrest has highlighted long-standing 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.

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

U.S. Secretary of State Hillary Clinton has said she is deeply concerned about the situation. She has called for a transparent investigation into the violence and said the situation underscores the need for “serious efforts to achieve national reconciliation in Burma.”