With tens of thousands fleeing Syria every month, the number of refugees worldwide in 2012 is set to be the highest this century, a senior United Nations official said on Monday.

Antonio Guterres, the body's High Commissioner for Refugees, told his UNHCR agency's executive committee that its ability to cope was being stretched to the limit.

"Already in 2011, as crisis after crisis unfolded, more than 800,000 people crossed borders in search of refuge -- an average of more than 2,000 refugees every day," the former Portuguese prime minister said.

That total had been the highest since the turn of the century "and so far this year more than 700,000 people have fled from the Democratic Republic of Congo, Mali, Sudan and Syria," Guterres said.

Last Friday, another UNHCR official said the total from Syria could reach 700,000 this year, nearly four times its earlier estimate as government troops battle rebels across the country.

About 294,000 refugees fleeing 18 months of fighting have already crossed into Jordan, Iraq, Lebanon and Turkey, or awaitregistration there, Panos Moumtzis told a news briefing.

He said 100,000 people had fled Syria in August, 60,000 in September and at the moment 2,000 or 3,000 were crossing daily into neighbouring countries.

The new refugees are joining some 42 million around the globe who have fled across borders to escape violence. Many of these have been in temporary shelter provided by the UNHCR for a decade or more, some for even longer.

Amid the global economic crisis and with budgets of governments stretched, Guterres told the executive committee that the cost of helping refugees was escalating fast while long-lasting crises like Afghanistan and Somalia continued.

"We are at a moment when the demands on us are rising while the means available to respond have remained at a similar level to last year," he said.

"Our operations in Africa, in particular, are dramatically underfunded. At this moment, we have no room for unforeseen needs, no reserves available. In today's unpredictable operating environment, this is a cause for deep concern."