Dozens of aid workers were under police protection in western Myanmar Thursday sparking UN and US alarm after mobs attacked the offices of international relief agencies in a new surge of violence.

Authorities tightened a curfew in Sittwe, the capital of volatile Rakhine State, after gangs of Buddhists ransacked the offices of aid groups who have faced accusations of bias in favour of local Muslims.

Police said they had escorted some 71 aid workers from nine organisations to safety — 32 foreigners and 39 Myanmar nationals.

The unrest erupted late Wednesday when hundreds of Buddhists massed around the offices of Germany-based medical aid group Malteser International. Police said the stone-throwing mob accused an American aid worker of handling a Buddhist religious flag in a disrespectful manner.

The United Nations said its offices had been attacked during the unrest, urging the government to do more to protect humanitarian workers, who have come under increasing pressure from Buddhist nationalists in Rakhine, where tensions are high following several outbreaks of deadly communal violence largely targeting Rohingya Muslims.

UN Resident and Humanitarian Coordinator in Myanmar Toily Kurbanov said he was “deeply concerned” about the violence, adding that the organisation was “determined” to continue operating in the region.

“I reiterate concern that any reduction of humanitarian presence could negatively affect the protection of vulnerable people,” he said. The United States Embassy in Yangon also issued a statement condemning the “mob violence” and confirmed that at least three of its citizens were among the aid workers given “emergency relocation”.

– Lack of adequate security –

“Despite some efforts by local authorities to ensure the security of humanitarian workers, we remain deeply concerned about the continued lack of adequate security forces and rule of law on the ground in Sittwe,” it said.

Some staff returned to Myanmar’s main city Yangon by plane following the incident, but around 50 people were sheltering at a police station on Thursday.

Police fired warning shots into the air to disperse the crowd, Police Lieutenant Colonel Min Aung told AFP, adding that groups of 40 or 50 people had roamed the city damaging houses rented to foreigners.

He said the situation had calmed as the dusk-to-dawn curfew took effect. Previously a curfew of midnight to 4am had been in force because of earlier unrest.

Another police official said the military had been brought in to control the situation. Local residents reported seeing troops on the streets in the largest numbers since 2012.

Rakhine state remains a tinderbox after festering animosity between local Buddhists and Muslims exploded into bloodshed in 2012, leaving around 140,000 people displaced and casting a shadow over the former junta-run country’s much-hailed reforms.

Buddhist flags have been hung across the city as part of protests against Muslims in the run up to a nationwide census this week that many fear could further inflame the situation in Rakhine.

Malteser Country Coordinator Johannes Kaltenbach said almost all humanitarian and UN offices and warehouses in Sittwe were damaged or looted by the mobs in what he said was violence “planned in advance” to stop international groups facilitating the census.

“This is an orchestrated effort trying to make the census in Rakhine impossible,” he told AFP, rejecting the claims that a Buddhist flag had been treated with disrespect, but confirming one had been removed from the group’s office building to preserve impartiality.

He said Malteser’s three foreign and five local staff were all sheltering at the Sittwe police station and did not fear for their safety. They will fly to Yangon on Friday.

Kaltenbach added that the group’s office windows had been smashed, computers destroyed and warehouses “ransacked” in the incident. Aid groups have come under increasing pressure in the state, with concerted campaigns by local nationalists who accuse them of being biased towards the Muslim Rohingya, who make up the vast majority of those who lost their homes in the unrest.

Last month medical aid group Doctors Without Borders (MSF) was ejected from the region following a series of protests, leaving tens of thousands of Rohingya in isolated communities reliant on government healthcare — a shortfall authorities insist they can meet.

At the time experts warned the move may embolden Rakhine hardliners to press for the expulsion of other foreign aid groups.

The Rohingya, referred to by the United Nations as one of the world’s most persecuted peoples, are subject to a web of restrictions on travel, work and even marriage.

They are viewed as illegal immigrants from neighbouring Bangladesh by local Buddhists and the government, who call them “Bengali”.