TRIPOLI, Lebanon -- A teenager from central Syria became the one millionth Syrian refugee to register in Lebanon on Thursday, a "devastating milestone" for the tiny Arab country with about 4.5 million people of its own, the UN refugee agency said.

Signing up for aid, 19-year-old Yahya recounted his long ordeal. After being trapped by the fighting for more than two years in his native city of Homs, he was evacuated earlier this year and travelled to Yabroud, a rebel held town near the Lebanese border that soon came under a crushing government offensive.

When staying there was no longer an option, he crossed into Lebanon with his mother and two sisters on March 8. Yahya's father was not with them -- he died from sniper fire in Homs in September 2011.

On Thursday, Yahya registered at the UNHCR centre in the northern Lebanese city of Tripoli.

"We didn't know where to go. We just wanted to get away from all the shelling and fighting," he said, giving only his first name for fear that his relatives back in Syria would be targeted.

The conflict in Syria, a country with a pre-war population of 23 million, has killed more than 150,000 people, according to the Britian-based Syrian Observatory for Human Rights, which documents the fighting through a network of activists on the ground.

The war has uprooted millions of Syrians from their homes, and the UN estimates there are now more than 2.5 million Syrians registered in neighbouring countries, with more than 47,700 more awaiting registration.

In addition to those, there are hundreds of thousands of Syrians who fled Syria and have not registered as refugees.

Neighbouring Turkey and Jordan, in addition to Lebanon, have taken in most of the refugees.

But three years after Syria's conflict started, Lebanon has become the country with the highest per-capita concentration of refugees recorded anywhere in the world in recent history, the UNHCR said.

"The number of refugees fleeing from Syria into neighbouring Lebanon surpassed 1 million today, a devastating milestone worsened by rapidly depleting resources and a host community stretched to breaking point," the agency statement said.

As a result, Lebanon is struggling to cope with a massive crisis that has become an unprecedented challenge for aid agencies.

Along with the social and economic strain of the refugees, Syria's sectarian war has also frequently spilled over into Lebanon with deadly clashes between factions supporting opposing sides in the fighting next door.

Militants from Lebanon's Shiite militant group Hezbollah are fighting alongside President Bashar Assad's forces in Syria while many among Lebanon's Sunni population support the rebels trying to topple him.

The 1 million Syrians are a huge burden for Lebanon, which has a 4.5 million-strong population, UNHCR said. The agency registers 2,500 new Syrian refugees daily in Lebanon -- more than one person per minute.

In addition to the registered refugees, there are tens of thousands of other Syrian refugees who are not registered and Lebanese officials estimate the number of unregistered refugees to be as high as 400,000.

"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," said Antonio Guterres, UN High Commissioner for Refugees in a statement. "Lebanon hosts the highest concentration of refugees in recent history."

For Yahya and his family, there was no other place to go than Lebanon. "We were looking around to go somewhere else in Syria but no place is safe," he said.

UNHCR said the influx from Syria is accelerating.

In April 2012, there were 18,000 registered Syrian refugees in Lebanon and by April 2013 they reached 356,000 and at the beginning of April this year they reached, 1 million.

The World Bank estimates that the Syria crisis cost Lebanon US$2.5 billion in lost economic activity during 2013 and threatens to push 170,000 Lebanese into poverty by the end of this year, the UNHCR statement said.