ISTANBUL — Syria’s humanitarian crisis is rapidly worsening and may be much larger than the United Nations. and major governments are describing it, according to diplomats and officials of U.N. organizations.

The Office of the U.N. High Commissioner for Refugees, the organization’s refugee agency, estimated last week that 360,000 Syrians had fled the country, but a UNHCR official told McClatchy that the number may be double that, more than 700,000, including 150,000 Syrians who’ve sought refuge in Egypt, a nation that shares no borders with Syria.

After months in which international aid groups wondered why there were so few refugees, the numbers are increasing fast. As of Friday, 280,000 Syrians had completed the registration process with UNHCR, more than 10 times the number on April 1.

How many people are displaced in Syria remains uncertain, bedeviling efforts to plan to assist them. In addition to those who’ve fled the country, the U.N. estimates that 1.2 million Syrians are in camps or other people’s homes, but still in the country. The Syrian government estimates the number at 3 million, and a respected Syrian opposition spokesman says it could be three times that.

The United States has offered no estimate of its own, saying it’s taking its lead from the United Nations.

“The crisis in Syria in humanitarian terms is very, very serious, certainly one of the top humanitarian priorities we have in the world,” Kelly Clements, a deputy assistant secretary of state, told McClatchy.

Clements said the administration hadn’t asked U.S. intelligence agencies to come up with an independent assessment of the humanitarian crisis. Producing a U.S. estimate “would not be something we’ve done in any crisis over decades of experience,” she said. ”We back up the (U.N.) Office for the Coordination of Humanitarian Affairs.”

Others question the U.N. estimate of internally displaced, noting that such people traditionally have been undercounted in conflicts because many stay in the homes of relatives and friends or in isolated areas. The U.N. now has a special rapporteur for internally displaced people and it makes use of assessments from a special monitoring group in Geneva. But with limited or no access, it can only make an educated guess.

“How can anyone come up with a figure in the country as it stands?” said Frank Smith, a spokesman for the Internal Displacement Monitoring Center, the group that helps the U.N. count such populations.

“We looked at patterns of movement, at population densities, and extrapolated from that. It is a soft figure,” he said. He called the U.N. figure of 1.2 million low and said his group now used 1.5 million. But that number, too, is likely to be inaccurate and a new assessment is under way, but it won’t be complete until early next year.

He said his organization had asked the Syrian government how it reached its estimate of 3 million.

“Winter can be quite cruel in parts of Syria,” he said. “People who have lost all their possessions, whose livelihoods are gone, will be increasingly beholden on humanitarian aid. The death toll from bombs and bullets is one thing. The death tolls from chronic diseases, child mortality, lack of prenatal and postnatal care, the old and the vulnerable, who won’t have access to the necessary warmth – it is a terrible situation.”

A prominent opposition spokesman, Mohammad Bassam Imadi, formerly the Syrian ambassador to Sweden, estimates the internally displaced as high as 10 million. Imadi, who stays in close touch with the rebel leadership in Syria, said he’d reached that figure by looking at the populations of major cities under bombardment and trying to guess how many residents remained.

In Aleppo, he said, only 500,000 of the city’s former population of 4 million remain in their homes. He estimated that half of the 6 million residents of greater Damascus have fled and that the population of Homs had been reduced to 100,000 from 1 million. In many other cities, he said, no more than half the inhabitants remain.

Imadi launched a new organization early this month to funnel aid to municipal authorities, making a presentation Oct. 2 before senior U.S. and other diplomats in Istanbul. He and several colleagues described the situation as catastrophic.

Other respected Syrian human rights figures, such as Radwan Ziadeh, of the Syrian Center for Political and Strategic Studies, said Imadi’s figure showed “some exaggeration.” Ziadeh estimated the displaced at 2.5 million.

Clements said the number of internally displaced people was “very dynamic,” and she preferred to focus on a U.N. estimate of the number of civilians it’s trying to reach, some 2.5 million, both displaced and just those in need of assistance.

“We tend, for inside Syria, to use the U.N. figure as definitive for now. That doesn’t mean it will not change,” she said.