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 neighboring Lebanon, Jordan, Turkey, Iraq and Egypt, which are all but buckling under the refugee burden.

"The world has a debt of gratitude to the neighboring 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 program.

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 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 neighbors since the conflict erupted in March 2011.

More than 30 humanitarian organizations meanwhile appealed Monday for countries to take in around 180,000 refugees from Syria's neighbors, 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".

Syria's neighbors 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 meager resources".

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