More than two million refugees have now fled Syria's civil war in human suffering unparalleled in recent history, the United Nations said on Tuesday.

Of the total Syrian population of about 20 million, either inside or outside the country, one third is displaced and almost half is in need of assistance, U.N. High Commissioner for Refugees António Guterres told a news conference.

"What is appalling is that the first million fled Syria in two years. The second million fled Syria in six months," he said.

The number of refugees stood at about 200,000 just a year ago, so the tide of men, women and children crossing borders has risen almost 10-fold over the past 12 months, figures from the U.N. refugee agency UNHCR showed.

"Syria has become the great tragedy of this century - a disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history," Guterres said.

"The only solace is the humanity shown by the neighbouring countries in welcoming and saving the lives of so many refugees."

However, the number of refugees has grown more slowly than UNHCR once feared. It previously projected the total would hit 3.45 million by the end of the year, based on border crossings in February-March that averaged about 8,000 per day.

So far in 2013, the daily figure has averaged between 5,000 and 6,000, which means the total is likely to be 2.6-2.7 million by the end of the year, Guterres said.

But UNHCR's figures include only officially registered refugees and those awaiting registration, and unofficial refugees probably number in the hundreds of thousands, he said.

The number of people displaced inside Syria is 4.25 million, in line with a forecast made in June.

The flow of Syrian refugees has piled pressure on neighbouring host countries, UNHCR said.

AID MEETING

Guterres will meet the foreign ministers of Iraq, Jordan, and Turkey and the minister of social affairs of Lebanon - the four main hosts of Syrian refugees - in Geneva on Wednesday to work out ways to raise more international aid.

Guterres declined to comment on the impact of possible U.S. air strikes on Syria, but said there were contingency plans in place for a surge in refugees, regardless of the cause.

"We all need to be prepared for things to get worse before they get better."

UNHCR said last month its work had so far stopped the refugee crisis spiralling out of control.

But "a far more substantial and coherent strategy" was needed than the $2.9 billion refugee aid effort already underway, it said then.

Syria's uprising against four decades of rule by the family of President Bashar al-Assad has turned into an increasingly sectarian civil war that has killed more than 100,000 people.

The conflict has taken a heavy toll on Syria's children, 39 percent of whom have dropped out of school in the past academic year, according to the U.N. children's agency UNICEF.

More than 3,000 schools have been damaged or destroyed and almost 1,000 are being used to house displaced people.

Sweden said on Tuesday it had decided to offer all Syrian refugees in the country permanent residency. Swedish news agency TT said Sweden was the first country in the EU to do so. Some Syrians in Sweden had to date been granted temporary residence.

The Scandinavian country has received almost 15,000 Syrian refugees in 2012 and 2013. Its Migration Board said the security situation in Syria was "extreme". – Reuters