UN refugee agency chief Antonio Guterres warned Monday that a surge in the number of refugees around the world was causing a crisis that was unprecedented in recent history.

"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, and this was higher than at any time in the last decade," Guterres said in an opening address to the annual UNHCR Executive Committee.

And the situation is getting worse, he said, with more than 700,000 people have fled from the Democratic Republic of Congo, Mali, Sudan and Syria so far this year.

He said the cost of helping the world's more than 42 million forcibly displaced people was also escalating fast due to drawn-out and large-scale displacements in places like Afghanistan and Somalia.

"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, pointing out that UNHCR operations in Africa were especially underfunded.

"At this moment, we have no room for any unforeseen needs. No reserves available. In today's unpredictable operating environment, this is a cause for deep concern," he said.

The UNHCR's Executive Committee, made up of 87 member states, is set this week to review its annual budget of $3.92 billion for 2013 and to look over the organisation's programmes.

Guterres appealed to committee members to do more to help address the needs of "all those who find themselves uprooted from their homes and communities, as the crises of today and tomorrow continue to unfold."