United Nations aid agencies said on Thursday that a $4.5 billion appeal to tackle the Syrian refugee crisis in 2015 was less than a quarter funded, putting millions of vulnerable people at risk, and had already led to cuts in vital assistance.

The shortfall has meant 1.6 million refugees have had their food assistance cut this year and 750,000 children are not attending school, the agencies and partner organizations said, calling on countries to deliver on their pledges.

"We are so dangerously low on funding that we risk not being able to meet even the most basic survival needs of millions of people over the coming six months," U.N. High Commissioner for Refugees Antonio Guterres said in the statement that represented more than 200 groups involved in the appeal.

The refugee response plan is a $5.5 billion appeal with $4.5 billion earmarked for U.N. agencies and nongovernmental organizations. An additional $1 billion is intended to help regional countries with refugee populations.

Only $1.06 billion of the money for the agencies and nongovernmental organizations had been received by the end of May, leaving a $3.47 billion funding gap, the statement said. It did not specify which donors had failed to deliver on their pledges."If more funds are not forthcoming soon, up to 130,000 vulnerable families will not be provided with cash assistance to help them meet their basic needs and vulnerable people will stop receiving their monthly food vouchers altogether," it said.

It warned that up to 1.7 million people might face winter this year without fuel, shelter, insulation, blankets or warm clothes. Last year, refugees, including children, died during an especially fierce snowstorm that hit the region.