Nearly 90,000 Rohingya have fled to Bangladesh since violence erupted in Myanmar in August, pressuring scarce resources of aid agencies and communities already helping hundreds of thousands of refugees.
The violence in Myanmar's northwestern Rakhine state was triggered by an attack on dozens of police posts and an army base by Rohingya militants on 25 August.
The ensuing clashes and a military counter-offensive have killed at least 400 people.
Myanmar officials blamed Rohingya militants for the burning of homes and civilian deaths but rights monitors and Rohingya fleeing to neighbouring Bangladesh say the Myanmar army is trying to force Rohingya out.
The treatment of Buddhist-majority Myanmar's Muslim Rohingya is the biggest challenge facing leader Aung San Suu Kyi, accused by Western critics of not speaking out for the minority that has long complained of persecution.
Myanmar says its security forces are fighting a legitimate campaign against "terrorists" responsible for a string of attacks on police posts and the army since last October.
The number of those crossing the border into Bangladesh surpassed the number who escaped Myanmar after a series of much smaller militant attacks in October that set off a military operation beset by accusations of serious human rights abuses.
The newest estimate, based on calculations by UN workers in the Bangladeshi border district of Cox's Bazar, takes to nearly 150,000 the total number of Rohingya who have sought refuge in Bangladesh since October.
"We are trying to build houses here, but there isn't enough space," said Mohammed Hussein, 25, who was still looking for a place to stay after fleeing Myanmar four days ago.
"No non-government organisations came here. We have no food. Some women gave birth on the roadside. Sick children have no treatment."
An unofficial camp for Rohingya refugees that sprang up after the October attacks is being dramatically expanded.
Among new arrivals, about 16,000 are school-age children and more than 5,000 are under the age of five who need vaccine coverage, aid workers said over the weekend.
The number of unaccompanied children was high and many were "traumatized and hungry", they said.
Bangladesh officials have said at least 53 bodies have either washed up on Bangladeshi shores or have been found in the Naf River, which separates the two countries, many with bullet or knife wounds.
In addition to tens of thousands of Rohingya, more than 11,700 "ethnic residents" had been evacuated from northern Rakhine state, the Myanmar government has said, referring to non-Muslims.
Myanmar state media on Monday carried stories showing authorities providing food and medical care to the evacuees.
The army said on Sunday Rohingya insurgents had set fire to monasteries, images of Buddha as well as schools and houses in the north of Rakhine state. It posted images of destroyed Buddha statues.
A Myanmar military source said security forces still had difficulty penetrating the remote northern part of Maung dawregion - close to the Bangladeshi border.
He said that the area between the state capital of Sittwe and the major regional town of Maungdaw, where the bulk of Aug.25 attacks took place, was under military control
