The refugee agency said 6.5 million people are displaced within Syria, with a record 3 million people qualifying as refugees.
"Almost half of all Syrians have now been forced to abandon their homes and flee for their lives," UNHCR said in a statement Friday. "One in every eight Syrians has fled across the border, fully a million more than a year ago. A further 6.5 million are displaced within Syria. Over half of those uprooted are children."
The agency said families are in dire straits by the time they flee Syria, many having spent a year or more on the run inside the country before making a 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," the statement said.
"Refugees crossing the desert into eastern Jordan are being forced to pay smugglers hefty sums (US$100 a head or more) to take them to safety."
Most of the refugees have fled to nearby countries, including 1.14 million to Lebanon, 815,000 to Turkey, and 608,000 to Jordan.
The statement called the Syria crisis the "largest in UNHCR's 64-year-history."
It "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 António Guterres, the U.N. High Commissioner for Refugees. "But the bitter truth is that it falls far short of what's needed."
Donors have given $4.1 billion to UNHCR since 2012, the agency said, but another $2 billion will be needed before the end of the year.