UN High Commissioner for Refugees Antonio Guterres said the international community needed to support a $1 billion plan, pushed by his and other international groups, to address the needs of those displaced by the Syrian conflict.
The UN on Wednesday stated that the number of people fleeing the fighting in Syria could reach 1.1 million by June.
The humanitarian situation in Syria is "the most dramatic crisis we are facing today," he said, adding developed nations, including those in Europe, needed to accept more refugees, as Turkey, Jordan, and Lebanon have done.
Guterres said the refugee problem in Syria is getting worse as the government of President Bashar al-Assad and opposition groups continue their bloody battles for the control of the energy-rich nation.
In April last year, the UNHCR recognised a total 33,000 Syrian refugees, but the agency is now registering an additional 35,000 people every week, he said.
"That puts enormous pressure on the UNHCR's capacity and resources to deliver protection and assistance," he said.
But, he acknowledged political leaders may find it hard to convince electorates of the need to act when faced with a multitude of demands on their sympathies.
"Last year, simultaneously, we had four refugee crises. Mali, Sudan/South Sudan, Syria and Democratic Republic of Congo," he told a press conference in Tokyo.
"Inevitably... there is certain fatigue because crises are so many. There is not capacity in public opinion to follow them with the same kind of interest," he said.
But, he warned, each was important.
"All these crises are creating hundreds of thousands of refugees, at the same time, they correspond to... global threats to peace and security," he said.
The former Portuguese prime minister stressed that helping refugees and stabilising the resource-rich but unsettled nations in Africa and the Middle East, was in the economic interests of rich countries, including Japan.
Guterres was making a routine visit to Japan to ask for its continued support, most of it financial, for his agency.