Twenty-eight countries have agreed to take in more than 100,000 refugees from Syria, doubling the number of migrants to whom they were initially offering asylum, the United Nations refugee agency, UNHCR, has said.

Antonio Guterres, the head of the UN agency, said on Tuesday that he was pleased with the new pledges but that much more help was needed to curb the rampant displacement crisis in Syria.

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

He said 28 countries had expressed solidarity with Syrians who fled their war-ravaged country, and with neighbouring Lebanon, Jordan, Turkey, Iraq and Egypt, which are struggling to cope with a bulk of the refugees totalling in the millions.

The world has a debt of gratitude to the neighbouring countries that probably we will never be able to fully pay or to fully express," Guterres old delegates.

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.

However, the total pledges to help refugees 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 about 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 on Monday for countries to take in about 180,000 refugees from Syria's neighbours, while Guterres acknowledged a full 10 percent, or nearly 300,000, of the refugees were in need of resettlement.

'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 that the 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 on 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.

'Europe-wide campaign needed'

Germany, which is hosting about 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 news agency.

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 about 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.