The number of refugees who have fled Syria and registered in Lebanon has surpassed the "devastating milestone" of one million, the UN says.
Lebanon now has "the highest per capita concentration of refugees worldwide", said the head of the UN refugee agency.
"For Lebanon, a small nation beset by internal difficulties, the impact is staggering," he added.
About 9.5 million people, almost half of Syria's population, have fled their homes since the start of the conflict.
More than 2.5 million have fled the country, with large numbers being taken in by Turkey, Jordan, Iraq, Egypt and others.
However Lebanon is bearing the biggest burden of all - the number of Syrian refugees there now amount to a quarter of the local population.
The BBC's Jim Muir in Beirut says this is a huge strain for Lebanon, the smallest and most vulnerable of Syria's neighbours.
Last month the Lebanese foreign minister said the crisis was "threatening the existence" of his country.
"The Lebanese people have shown striking generosity, but are struggling to cope," Mr Guterres said.
The UN's refugee agency, UNHCR, notes that the influx of refugees is accelerating.
"Every day, UNHCR in Lebanon registers 2,500 new refugees: more than one person a minute," the agency said in a statement.
International aid agencies are finding it increasingly difficult to cope with the flow of refugees.
UNHCR has only received 14% of the $6.5bn (£4bn; 4.7bn euros) funding it has asked for.
The hardships many refugees are facing was dramatically highlighted last week when a mother with a sick husband and four children set fire to herself in protest at not receiving help. She was critically injured.
The civil war in Syria between Sunni-led rebels and the government of Bashar al-Assad has also fuelled sectarian tensions in Lebanon, which has a large Sunni and Shia Muslim population.
Hundreds of people in Beirut and other parts of the country have been killed in violence between opponents and supporters of President Assad.