Story highlights U.N. staff in Lebanon register 2,500 new Syrian refugees every day

The refugees now make up almost a quarter of the resident population

"For Lebanon ... the impact is staggering," a U.N. official says

Lebanon has the highest per capita concentration of refugees in the world

The number of Syrian refugees in Lebanon has passed 1 million, the United Nations' refugee agency said Thursday, making up almost a quarter of the country's resident population.

"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," U.N. High Commissioner for Refugees Antonio Guterres said in a statement.

Their numbers have made Lebanon the country with the highest per capita concentration of refugees in the world, the agency said.

The total number of registered Syrian refugees in all countries is 2.58 million, according to the United Nations. Other nations with large populations of Syrian refugees include Jordan and Turkey.

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Syrian refugees: By the numbers

The number in Lebanon has now risen into seven figures, from just 18,000 two years ago.

The United Nations has said that more than 100,000 people, many of them civilians, have been killed in Syria since a popular uprising spiraled into a civil war in 2011.

U.N. staff in Lebanon register 2,500 new Syrian refugees every day, the UNHCR said.