The number of Syrian refugees who have fled to Lebanon has officially topped 1m.

It highlights the growing humanitarian catastrophe caused by Syria's civil war and the huge burden placed on its poorly prepared neighbours.

The United Nations refugee agency UNHCR marked what it called a devastating milestone by formally registering an 18-year-old student from the city of Homs as the millionth refugee at a ceremony in Lebanon's Mediterranean city of Tripoli.

Three years of conflict in Syria were sparked by protests against President Bashar al-Assad's autocratic rule.

The war has caused one of the greatest upheavals seen in the Middle East - and one which shows no sign of abating.

With a population of just 4m, Lebanon now has the highest per capita concentration of refugees in the world.

The influx has been described by the government as an existential threat in a country scarred by its own volatile history.

School-aged refugees eclipse the number of Lebanese children in the country's state schools, the UN says, and 2,500 new refugees are registered every day.

"The extent of the human tragedy is not just the recitation of numbers," UNHCR representative Ninette Kelley told reporters in Tripoli.

"Each one of these numbers represents a human life who ... have lost their homes, their family members, their sense of future."

Syrians have also fled to Turkey, Iraq, Jordan and Egypt.

The official total of 2.6 million Syrian refugees, which understates the scale of the exodus, means Syrians will soon overtake Afghans as the world's biggest refugee population.

Many millions more have been displaced inside Syria, and the pace has only accelerated in the last 12 months.

In April 2013, two years after the Syrian crisis erupted, there were 356,000 refugees in Lebanon. That number has nearly tripled in the last 12 months.

"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," UN High Commissioner for Refugees António Guterres said in a statement.