A 46-year-old refugee in Lebanon described the plight of his family, who has spent the past year living in an old sheep shed.
"I cry in my heart. I feel depressed. It's unjust. Is there a worse way to live than this?" the refugee, identified as Ahmed, told the aid group.
"Our situation is terrible to the maximum. We didn't expect there were humans who could live the way we are living."
Host countries: Resources pushed to the limit
The endless flood of refugees has drained resources in neighboring countries in unpredictable ways.
Lebanon's population has increased up to 10% because of Syrian refugees, the U.N. agency said.
Jordan's energy, water, health and education services have been severely strained.
Turkey has spent more than $600 million setting up 17 refugee camps, with more under construction.
And Iraq, already grappling with 1 million internally displaced citizens, has taken in more than 100,000 Syrian refugees in the past year.
"We are doing everything we can to help, but the international humanitarian response capacity is dangerously stretched," said Guterres, the U.N. high commissioner for refugees. "This tragedy has to be stopped."
Dozens of countries and groups around the world pledged more than $1.5 billion in humanitarian assistance for Syrians in January. But only $200 million had been received by the end of February, the United Nations said.
The war: Shifting but endless
While the end of the war seems nowhere in sight, the opposition has made notable strides against government forces.
This week, opposition fighters said they captured the major northern city of Raqqa -- the first time rebels have seized a provincial capital.
State-run media responded by saying government forces were battling suspected members of the radical Islamist group al-Nusra Front in Raqqa.
But footage from the city painted a different picture. Children climbed atop a fallen statue of former President Hafez al-Assad -- the current president's father -- and beat the cracked figure with a bat and shoes.
Some attribute the rebels' recent gains to shipments of arms from other countries.
But rebels are still fighting to wrest the largest city, Aleppo, from government hands. They recently took over a military base near the city, but full control of Syria's economic hub remains elusive.
While Damascus remains a regime stronghold, rebel and regime forces are battling very close to the city -- threatening to take the fight to the seat of President Bashar al-Assad's power.
Rebels detain U.N. peacekeepers