Violence in Myanmar has led to 370,000 people belonging to the Muslim Rohingya minority fleeing to Bangladesh since 25 August, up from a previous estimate of 313,000, the UN International Organisation for Migration said.
"The system is clearly at full stretch and needs all the support it can get," IOM chief spokesman Leonard Doyle said.
He declined to say how big he thought the exodus could get.
Rohingya militants attacked police posts in Rakhine state in late August, prompting a military backlash.
International pressure on Myanmar's government has increased as UN rights chief Zeid Ra'ad Al Hussein said the violence seemed to be a "textbook example of ethnic cleansing".
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"Clearly the estimates [of refugee numbers] have been bypassed several times over," Mr Doyle told a briefing in Geneva.
"I’m reluctant to give a number but obviously people fear that it could go much higher."
Meanwhile, China has said it backs the Myanmar government's efforts to "uphold peace and stability" in Rakhine state.
But UN diplomats have said China, one of Myanmar's main trade partners, has been resisting involvement by the top UN council in addressing the crisis.
"We condemn the violent attacks which happened in Rakhine state in Myanmar," China's Foreign Minister Geng Shuang told a regular news briefing.
"We support Myanmar's efforts in upholding peace and stability in the Rakhine state. We hope order and the normal life there will be recovered as soon as possible," he said.
"We think the international community should support the efforts of Myanmar in safeguarding the stability of its national development."
