A senior UN official has said an estimated $200 million would be needed to help Rohingya refugees in Bangladesh for six months.
Bangladesh and aid organisations are struggling to help over 422,000 Rohingya Muslims who have arrived since 25 August, when attacks by Rohingya militants triggered a Myanmar crackdown that the United Nations has branded as ethnic cleansing.
Aid workers fear a humanitarian crisis is also unfolding in Rakhine state, though Myanmar has restricted access.
Given the "massive numbers" arriving in the past few weeks, the United Nations was expected to launch an appeal for $200million to help them for the next six months, an official said.
"It has not been confirmed, but it is a ballpark figure, based on the information we have," Robert D. Watkins, UN resident coordinator in Bangladesh said.
Mr Watkins said the situation had not stabilised in terms of new arrivals so it was difficult to say how many people to plan for, or how long.
"We don’t want to plan a 10-year operation, obviously, because we want to maintain hope that there will be a way for negotiating a return of the population," he said.
He added: "We can’t plan too far in the future, because it becomes a self-fulfilling prophecy ... politically, it sends a strong signal, which we don’t want to send, which is that people are going to be here for a long time.
"And our donors are not prepared to respond to anything beyond a one-year time frame, given the massive amounts of money we are asking for."
Aid groups in Bangladesh have warned of a public health disaster unless help is increased massively.
"We need to scale up quickly," a World Health Organisation representative in Bangladesh said.
"If we don't drastically improve water and sanitation we will face water-borne diseases including cholera."
The United States wants Myanmar to take urgent action to end the violence in Rakhine state, where a military offensive has created a crisis that could jeopardise its economic and political transition.
"We think, urgently, actions need to be taken to stop this violence and facilitate humanitarian assistance, lower the rhetoric, lower the tension and start doing the hard work to solve the longer-standing problems," US Deputy Assistant Secretary of State Patrick Murphy said.
Myanmar leader Aung San Suu Kyi has faced a barrage of international criticism over the plight of the Rohingya, for not speaking out more forcefully against the violence, or doing more to rein in security forces over which she has little power.
Mr Murphy said there were "many points of responsibility" and he wanted to see everyone follow through on commitments Ms Suu Kyi made to uphold rights and the law in an address to the nation on Tuesday.
"There is the elected government, there are the security forces which have authorities that don’t fall under the purview of the civilian elected government, there are local leaders and there is the broader population, among which there are many emotions and many tensions," he said.
"Significant responsibility sits with security authorities and local officials in Rakhine state and we are looking for their cooperation to make these commitments a reality," Mr Murphy said.
Myanmar dismisses accusations of ethnic cleansing, saying it has to tackle the insurgents, whom it accuses of setting fires and attacking civilians as well as the security forces.
While the United States has urged action to halt the violence, China, which has close economic and political ties with Myanmar, has welcomed measures by the government to alleviate the situation in Rakhine state.
