A "devastating milestone" has been reached with one million Syrian refugees now in Lebanon which has had a "staggering" impact on the country, the United Nations said.
UN High Commissioner for Refugees Antonio Guterres said: "The influx of a million refugees would be massive in any country. For Lebanon, a small nation beset by internal difficulties, the impact is staggering.
"The Lebanese people have shown striking generosity, but are struggling to cope.
"Lebanon hosts the highest concentration of refugees in recent history. We cannot let it shoulder this burden alone."
Refugees from Syria now equal one quarter of Lebanon's population, according to UN figures.