The United Nations has said more than three million Syrians have fled the country to escape the war.

The UN refugee agency says the Syrian crisis is the biggest humanitarian emergency of our era but the world is failing to meet the needs of the refugees.

The vast majority remain in neighbouring countries, with the highest concentrations in Lebanon Turkey and Jordan.

The host governments estimate that hundreds of thousands of Syrians have sought sanctuary in their countries without formally registering, the UNHCR said.

Increasing numbers of families arrive in a shocking state, exhausted, scared and with their savings depleted, it said.

"Most have been on the run for a year or more, fleeing from village to village before taking the final decision to leave.

"There are worrying signs too that the journey out of Syria is becoming tougher, with many people forced to pay bribes at armed checkpoints proliferating along the borders.

Refugees crossing the desert into eastern Jordan are being forced to pay smugglers hefty sums (ranging from $100 per person or more) to take them to safety," it added.

More than 190,000 have been killed in Syria's three-year civil war.