The number of refugees who have fled Syria and registered in Lebanon has exceeded 1 million, the UN has said.

Refugees from Syria, half of them children, now equal a quarter of Lebanon's resident population, the UN high commissioner for refugees said, warning that most of them lived in poverty and depended on aid for survival.

The UN agency said the figure was "a devastating milestone worsened by rapidly depleting resources and a host community stretched to breaking point".

Lebanon has become the country with "the highest per-capita concentration of refugees worldwide", and is struggling to keep pace, the statement 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 UNHCR chief, António Guterres, said.

The statement came amid reports that several mortar shells had struck the Syrian capital, Damascus, on Thursday, killing at least six people. Syrian state television said the heaviest blow had struck the suburb of Harasta, where six children had been killed and eight wounded. Five more were injured when another mortar shell struck the upscale neighbourhood of Maliki.

The Britain-based Syrian Observatory for Human Rights said shells had also struck central Damascus, including the Ummayad Square, close to TV and radio stations as well as the army command, but there were no casualties. There were also reports of heavy clashes and air raids in the Damascus suburb of Mleiha.

Syria's three-year war has killed more than 150,000 people, according to the Syrian Observatory for Human Rights, while half of the population are estimated to have fled their homes.

Of those who have fled Syria, nearly 600,000 have registered as refugees in Jordan and about 670,000 in Turkey.

Guterres said: "Lebanon has experienced serious economic shocks due to the conflict in Syria," while security has deteriorated as a result of rising regional instability.

The influx has put severe strains on Lebanon's health and education sectors as well as electricity, water and sanitation services.

"The Lebanese people have shown striking generosity, but are struggling to cope. Lebanon hosts the highest concentration of refugees in recent history," Guterres said.

He urged the global community to boost its support for Lebanon. "International support to government institutions and local communities is at a level that, although slowly increasing, is totally out of proportion with what is needed," Guterres said.

He said the humanitarian appeal for Lebanon was only 13% funded.

Half the refugees are children, the UNHCR report said.

"The number of school-aged children is now over 400,000, eclipsing the number of Lebanese children in public schools. These schools have opened their doors to over 100,000 refugees, yet the ability to accept more is severely limited," it added.

The vast majority of Syrian refugee children, however, are out of school.

Because of the dire economic situation their families endure, many children work. "Girls can be married young and the prospect of a better future recedes the longer they remain out of school," it said.