GENEVA (AFP) - Countries have more than doubled the number of Syrian refugees they are willing to resettle to over 100,000, the head of the UN refugee agency said Tuesday.

"We estimate (there) will be more than 100,000 opportunities for resettlement and humanitarian admission," Antonio Guterres told reporters after a high-level pledging conference in Geneva.

He said 28 countries had expressed solidarity with the millions of Syrians who have fled their war-ravaged country, and with neighbouring Lebanon, Jordan, Turkey, Iraq and Egypt, which are all but buckling under the refugee burden.

"The world has a debt of gratitude to the neighbouring countries that probably we'll never be able to fully pay or to fully express," Guterres had told delegates, urging them to do more.

There was no clear overview over which countries had pledged what, but Guterres hailed the roles played by Germany, Sweden and the United States in the resettlement programme.

While Tuesday's pledges more than doubled the some 40,000 spots available for resettlement before the conference, they still fall short of a UNHCR's target and the numbers experts say are necessary.

The UNHCR has called on countries by 2016 to help resettle some 130,000 of the more than 3.2 million registered refugees amassed in Syria's neighbours since the conflict erupted in March 2011.

More than 30 humanitarian organisations meanwhile appealed Monday for countries to take in around 180,000 refugees from Syria's neighbours, while Guterres acknowledged a full 10 percent, or around 300,000, of the refugees were in need of resettlement.

Tuesday's pledges, he said, were "not the end of the process (but) the beginning of a process".

'TERRIBLE PLIGHT'

Syria's neighbours warned the conference that the flood of refugees was tearing apart the fabric of their societies.

Lebanese Social Minister Rachid Derbas pointed out that, with 1.1 million refugees and counting, one in three people in his small country is now Syrian.

"The alarm bells ring unrelentingly," he said, decrying "the terrible plight" of Lebanese and Syrians alike amid job shortages, soaring prices, strained infrastructure, overcrowded schools and hospitals, on top of the insecurity spilling over from Syria's civil war.

Jordanian Interior Minister Hussein Hazza Al Majali warned the some 630,000 refugees in his country were "stretching our meagre resources".

"Without international support for Jordan, it will be difficult for us to continue to carry this huge burden," he told the conference.

Guterres repeatedly hailed Syria's neighbours for their unflinching generosity, urging "massive support from the international community".

Only 66,254 of the pledged spots announced Tuesday were "firm and concrete", he acknowledged.

But he said 11 countries had also announced they were creating new resettlement programmes or would be expanding existing programmes.

Germany, which is hosting some 80,000 Syrian refugees, including 20,000 in the UN resettlement programme, did not immediately pledge additional spots, saying it was waiting for a signal from the European Union and demanding a "Europe-wide campaign" to help distribute the burden more evenly.

"Europe cannot keep its eyes shut," German Interior Minister Emily Haber told AFP.

EU commissioner for migration Dimitris Avramopoulos meanwhile pointed out that 34,000 resettlement places have so far been offered by EU member states, marking the "largest commitment in the history of EU resettlement efforts".

"Nevertheless, I strongly believe that we can and must do more," he told the conference, without making a firm pledge.

Sweden, which counts the most Syrian refugees compared to population size, with some 60,000 asylum seekers from the war-torn country, said it would double its current 1,500 resettlement spots.

Swedish Justice and Migration Minister Morgan Johansson said he was proud his country was taking a leading role, but stressed the need for more countries to follow.

"The international community needs to step up to the plate," he told the conference.

Guterres also hailed the United States for vowing to "accelerate" its resettlement programme, which is already reviewing 9,000 Syrian cases.

"We are receiving roughly 1,000 new ones each month, and we expect admissions from Syria to surge in 2015 and beyond," Anne Richard, the US Assistant Secretary of State for Population, Refugees and Migration, told the conference.