The UN has decided to temporarily relocate non-essential staff in Burma's Rakhine state, where tension remains high amid unrest that has killed seven.
It said "serious disturbances" and the imposition of a state of emergency prompted the move, which is being implemented on a voluntary basis.
Violence flared after the murder of a Buddhist woman last month, followed by an attack on a bus carrying Muslims.
Bangladesh said it had stopped dozens of people trying to flee Burma by sea.
'Security and stability'
The UN released a statement saying it had decided "to temporarily relocate, on a voluntary basis, non-essential international and national" UN staff, affiliated organisations and their families.
The statement said it had requested full government support for the safety and security of all its staff and their families in Maungdaw, Buthidaung and Sittwe while they are relocated to the main city, Rangoon.
Official media said the state of emergency, imposed late Sunday night, was in response to increasing unrest and attacks and "intended to restore security and stability to the people immediately".
According to a Reuters report, the violence over the weekend began on Friday in the town of Maungdaw, spreading to state capital Sittwe and neighbouring villages.
Rival Buddhist and Muslim groups were seen setting houses on fire, reports said.
In a televised speech, President Thein Sein said the violence could put the country's moves towards democracy in danger.
Neighbouring Bangladesh has increased its security presence on the border amid fears of an exodus.
The Border Guard Bangladesh force on Monday said it had turned away a number of boats carrying refugees. Reports of the number of people ranged from 50 to 300.
The clashes in Burma began on 4 June when a mob attacked a bus in Taungup, Rakhine province, apparently mistakenly believing some of the passengers were responsible for the earlier rape and murder of a Buddhist woman.
Rakhine state is named after the ethnic Rakhine Buddhist majority, but also has a sizeable Muslim population, including the Rohingyas.
The Rohingyas are a Muslim ethnic group and are stateless, as Burma considers them to be illegal immigrants from Bangladesh.
A nominally civilian government was elected in Burma in 2010 and, in April this year, opposition politicians led by Aung San Suu Kyi entered Burma's parliament following historic by-elections.
However, the government is still dominated by the military and concerns over political repression and human rights abuses continue.