ISTANBUL -- The top U.S. aid official said Tuesday that Syria faces an immediate humanitarian emergency and that international plans to feed and support millions of destitute civilians have fallen short.

Rajiv Shah, administrator of the U.S. Agency for International Development, said that at least a million Syrians, forced from their homes by the national uprising and government bombing, would not have food and other vital basic support, and the number could be double that or more.

“The situation is now an emergency,” Shah told reporters during a conference call after visiting a refugee camp just inside Turkey’s border with Syria.

“Nearly 2.5 million people displaced from their homes require immediate support,” Shah said. But “global aid efforts are now reaching only 1.5 million.”

Meanwhile, a Turkish aid group that delivers food, tents and blankets to Syrians just across the border has warned of the danger of mass starvation in Syria this winter. Shah would not go that far but he acknowledged that his agency is preparing for “some very bad situations.”

He said the number of displaced – families that have been forced to move to the homes of friends or relatives, into public buildings or are living in tents or in the open – is “likely to be higher than 2.5 million.”

“Whether it’s 2.5, 3 or 4 million, there is a current shortfall,” he said.

Other estimates suggest that after 20 months of upheaval, the number of internally displaced could be far higher in the nation of 22 million. At least 750,000 Syrians have sought refuge abroad, according to officials of the U.N. High Commission for Refugees.

Earlier this month, Turkey’s Human Rights and Freedoms and Humanitarian Relief Foundation warned that 10 million Syrians could go hungry this winter. The foundation is the principal Turkish charity providing aid across the border to Syria, according to Syrian aid volunteers, who say it supplies truckloads of food daily for Syrians living in the tent city close to the border as they wait for space in Turkish refugee camps.

The group warned on its website Nov. 9 that commercial life had come to a standstill in Syria and that food production had been affected deeply by the displacement of at least 2.5 million Syrians. Osman Atalay, the foundation’s director of operations, warned that a shortage of medicine, food, heating oil and diesel fuel would lead to hunger and possible outbreaks of epidemics.

“For 20 months, the world public has spoken of a Syrian political crisis,” he told McClatchy in an interview after posting the warning. “In fact, the humanitarian tragedy and the trauma on the people have been passed over.” He said he derived his 10 million estimate from Syrians who come to the border and request flour, yeast and other foodstuffs. “We receive dozens of phone calls from Syrians for food aid every day,” he said in a follow-up email.

The Human Rights and Freedoms and Humanitarian Relief Foundation has achieved fame – and in some quarters notoriety – for having sponsored the Mavi Marmara aid ship that tried to break Israel’s blockade of the Gaza Strip in 2009. A Turkish court is currently trying four Israeli ex-commanders in absentia for Israel’s armed intervention, which left nine Turkish activists dead.