"Before the war, there were 560,000 Palestine refugees. We estimate that currently about 110,000 to 120,000 have left the country," UNRWA chief Pierre Krahenbuhl said on a visit to Damascus.
"There are about 45,000 who went to Lebanon, 15,000 to Jordan," he said.
"The others –- therefore almost half of those who have left –- have travelled, we presume, through Turkey and then to a variety of other countries.
"Some of them will be in Europe. We know of Palestine refugees who have reached parts of Asia. We know of some who have reached Latin America."
Syria is home to 12 refugee camps, three of them unofficial, according to UNRWA.
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Before the war, some 160,000 mostly Palestinians and Syrians lived in the district of Yarmuk in southern Damascus.
But the once thriving suburb has been devastated by conflict since late 2012 and UNRWA cannot access the camp to distribute aid to some 6,000 remaining residents.
"UNRWA will not give up its efforts to try to find ways to have this type of access in the future but in the meantime we are... concentrated on providing the assistance that we can in the neighbourhoods directly beside Yarmuk," Krahenbuhl said.
"The situation for Palestine refugees and civilians inside Yarmuk remains extremely desperate -- very, very difficult."
Last month, the Islamic State group had almost evicted rival Al-Qaeda jihadists from Yarmuk, according to a Palestine Liberation Organisation official.
Syria has hosted hundreds of thousands of Palestinians since their ancestors fled their homes when the state of Israel was created in 1948, and in successive conflicts since.