The United Nations refugee agency says the number of Syrian refugees in Lebanon has exceeded one million, calling it a "devastating milestone" for the tiny Arab country with depleted resources and an explosive sectarian mix of its own.

Three years after Syria's conflict started, Lebanon has become the country with the highest per-capita concentration of refugees worldwide, struggling to cope with a massive crisis that has become an unprecedented challenge for aid agencies.

Also, Syria's sectarian war has frequently spilled over into Lebanon, with deadly clashes between factions supporting opposing sides in the fighting next door.

The UNHCR said Thursday the one million Syrians are a huge burden for Lebanon, which has 4.5 million people.

The agency registers 2,500 new refugees daily in Lebanon — more than one person per minute.

Map: Where Syria's refugees have resettled

Note: Figures current as of April 2. There are also 19,697 Syrian refugees in North Africa.

Source: UNCHR Syrian Regional Refugee Response