GENEVA, Switzerland - The United Nations refugee agency Friday said one in every eight Syrians has fled the war-torn nation, raising the number of people fleeing the country to about three million over three years of the civil war.

There has been an increase of over one million refugees over the last year, the Geneva-based agency UN High Commission for Refugees said. More than half of all those uprooted are children, it said.

This, the agency said, has created a crisis that requires the biggest operation in its 64-year history.

"The Syria crisis has become the biggest humanitarian emergency of our era, yet the world is failing to meet the needs of refugees and the countries hosting them," said UN High Commissioner for Refugees Antonio Guterres.

Lebanon hosts 1.14 million Syrian refugees, the single highest concentration. Turkey has 815,000 and Jordan has 608,000.

The agency said 6.5 million of others have been displaced within Syria since the conflict began in March 2011. Syria had a prewar population of 23 million.

Donor nations have contributed more than $4.1 billion since 2012, but the agency says that an additional $2 billion is needed by the end of the year as the refugee population swells and temperatures fall in winter.

The new figures were revealed as there appears to be a surge in fighting, which is worsening the already desperate situation for Syrian refugees.

The extremist Islamic State group has expanded and is desperate to get its control of broad areas straddling the Syria-Iraq border.

"These include cities where populations are surrounded, people are going hungry and civilians are being indiscriminately killed," the agency said.

According to the UN agency, many of the new arrivals in Jordan come from the northern province of Aleppo and the northeastern region of Raqqa, a stronghold of the Islamic State, the militant group that has established caliphate in the areas it rules.

Angelina Jolie, the U.N's refugee agency special envoy, said international stability is steadily bleeding away in Syria. "Three million refugees is not just another statistic. It is a searing indictment of our collective failure to end the war in Syria," Jolie said in a statement following the report.

The UN estimates there are nearly 35,000 people awaiting registration as refugees, and hundreds of thousands who are not registered.

International Rescue Committee President David Miliband said the Syrian refugee crisis represents "three million indictments of government brutality, opposition violence and international failure".

"This appalling milestone needs to generate action as well as anger," he said, calling for more aid to Syria's overburdened neighbours and for civilians still in the country.