(Adds U.S. State Department comment, paragraphs 8-10)
By Aung Hla Tun
YANGON, March 27 (Reuters) - Protesters in Myanmar's Rakhine State opposed to a census attacked offices and houses used by international aid groups after reports a European staff member from one group had removed a Buddhist flag used as a symbol to boycott the operation, witnesses said.
The violence broke out late on Wednesday and continued into Thursday. Witnesses said security personnel fired warning shots in Sittwe, the capital of the western state, to disperse the attackers, who were demanding that the aid worker be handed over to them.
"A female European staff member allegedly took off a religious flag put up near her office by local people as a gesture of boycotting the government-sponsored census," Aung Mra Kyaw, an official from the Rakhine National Party, told Reuters.
"She wrapped it over her buttocks and threw it away when some eyewitnesses saw her yesterday evening."
It was not clear which aid agency she worked for. The offices of some United Nations agencies were also attacked, Aung Mra Kyaw said.
A Myanmar citizen working for one aid group said all staff from international non-governmental organisations and U.N. agencies were preparing to evacuate to the commercial capital, Yangon.
The authorities have extended a nightly curfew already in place to between 6 p.m. and 6 a.m. from midnight to 4 a.m.
In Washington, State Department spokeswoman Marie Harf said U.S. officials were "deeply concerned by mob violence" in Sittwe. Harf said the violence had "resulted in destruction of property and the emergency relocation of international aid workers, including at least three U.S. citizens, to safe havens."
Harf said the U.S. Embassy in Yangon has been in contact with officials in Myanmar about steps to reinforce security.
"Unhindered and regular humanitarian efforts to communities in need and the safety and security of humanitarian aid workers to ensure the effective delivery of these services is critical," Harf said.
At least 237 have been killed in religious violence in Myanmar since June 2012 and more than 140,000 have been displaced, many of them Muslim Rohingya in Rakhine State.
The government is about to embark on its first national census in more than 30 years.
The Rohingya are regarded as immigrants from neighbouring Bangladesh by many in Myanmar and are not recognised as one of the country's official ethnic groups, but some have said they will write in their ethnicity in a box on the census form.
That has stirred opposition to the whole census operation by some ethnic Rakhine people. (Additional reporting by Will Dunham in Washington; Editing by Alan Raybould, Ron Popeski and Cynthia Osterman)