ISTANBUL -- The first detailed survey of the humanitarian crisis in northern Syria suggests that the United Nations has grossly underestimated the number of civilians in dire need of assistance, a situation that experts say plays down the scope of the catastrophe.

“Syria is the largest IDP crisis in the world,” said Clare Spurrell of the Internal Displacement Monitoring Center, the leading body monitoring internally displaced people worldwide. “The longer we underestimate the reality of what is happening on the ground, the further we are getting from an appropriate response.”

The Office of the U.N. High Commissioner for Refugees released new figures Monday showing 2.08 million people in urgent need in six provinces of northern Syria. That’s way below a partial survey of the same provinces that the Syrian opposition and 10 international aid agencies conducted over four weeks in January.

That survey, undertaken by teams of researchers who met with local relief committees, religious leaders and local police, among others, estimated that the number of people in urgent need totaled at least 3.2 million in those provinces: Idlib, rural Aleppo, Latakia, Raqqa, Hasaka and Deir el Zour. That’s nearly three-quarters of the 4.3 million people thought to be living now in the surveyed areas of those provinces.

Of those, the survey found that 1.1 million are people who’ve been forced from their homes, making them dependent on others for food, shelter, health care and clean water.

And the situation is almost certainly worse than that: The researchers completed the survey in only about 40 percent of the provinces’ area and excluded the city of Aleppo, Syria’s largest, where fighting has raged since July. Once the survey is completed in the remaining 60 percent, the numbers are expected to go up.

“The size of the crisis is huge, much bigger than anyone had thought,” said Ghassan Hitto, the director of humanitarian relief for the Syrian Opposition Coalition, the collection of opposition groups that the United States and other countries have recognized as the sole legitimate representative of the Syrian people.

Hitto added that the figure of 3.2 million for those in urgent need of humanitarian aid in 40 percent of the six provinces “throws other numbers used for the entire country out of the water.” Nearly all of Syria’s 14 provinces are now engulfed in fighting.

The survey, whose results were compiled into an interim report titled “Joint Rapid Assessment of Northern Syria,” was released Jan. 27.

A spokeswoman for the U.N.'s Office for the Coordination of Humanitarian Affairs, Stephanie Bunker, said the U.N. was aware of the survey but didn’t take it into account in the numbers it released Monday. She said the UNHCR’s new figures came from the Syrian Arab Red Crescent, whose leadership is closely tied to the Syrian government.

Mark Bartolini, who retired at the end of last year as the director of the U.S. government’s Office of U.S. Foreign Disaster Assistance, said he believed that the U.N. was “grossly understating the numbers,” though he cautioned that the opposition-sponsored survey may overstate the situation.

“It’s never going to be perfect,” he said. “But in this case, it’s pretty far off.”

The United States says it relies on the United Nations’ numbers and has made no estimate of its own on the scope of the humanitarian crisis in Syria. Based on the current estimate of needs, the U.S. has provided $355 million for assistance.