More than half of the Syrian refugees in Lebanon are now living in extreme poverty, and the vast majority live below the poverty line, the UN's refugee agency said Friday.

According to the UN, more than a million Syrians have sought refuge in Lebanon since the war in their country erupted after anti-government protests in March 2011.

The massive influx has tested Lebanon, a country of just four million citizens that already struggled with overstretched resources before the arrival of Syrian refugees.

Over the last six years of the war, the refugee population has sunk further into debt and poverty, UNHCR said, with 58 percent of households now living in extreme poverty, defined as less than $2.87 per person a day.

That is an increase of five percent since last year, UNHCR said in an annual survey.

The survey found 76 percent of refugees were living below the poverty line, defined as less than $3.84 per person a day, and that nearly 90 percent of refugees were in debt.

"Syrian refugees in Lebanon are barely keeping afloat," said UNHCR's Lebanon representative Mireille Girard said.

"Most families are extremely vulnerable and dependent on aid from the international community."

One bright spot in the survey was a large jump in school enrolment of refugee children aged 6-14, with 70 percent now registered at school, up from around just half.

But the report found just 12 percent of adolescent refugees had finished their education.

More than 340,000 people have been killed in Syria since the conflict began, and over five million people have fled abroad as refugees.

Large parts of the country are now under government control after years of fighting, and there have been increasing calls in recent months in Lebanon for refugees to begin returning home, though aid agencies have said it is too early for returns to begin.