With an escalation in fighting that has prompted increasing numbers of Syrians to flee, "the humanitarian situation has changed significantly in the last four or five days," said an aid expert from the European Commission.

"It's like running behind a train that constantly keeps accelerating," he said.

While a current programme headed by the UN High Commissioner for Refugees provides for aid to 185,000 refugees from Syria by year's end, 120,000 have already registered with UNHCR and "the real number is much higher," he said.

"This will get worse before it gets better," the source added. "We need to scale up further... There is far too little money."

A $193-million appeal for Syrian refugees in Turkey, Jordan, Lebanon and Iraq is 80 percent under-funded and a $189-million plan to aid Syrians inside faces a 74 percent shortfall.

The EU experts said three out of four Syrian refugees were women and children who fled with only the clothes they were wearing.

Within the country, hundreds of thousands were on the move, some sleeping on the streets with nowhere to go. International donors plan to extend food aid to 1.5 million people compared to current aims of 850,000 people a month.

The EU appears to be by far the largest donor to humanitarian programmes, though Arab League nations were known to have handed "big cheques" to help Syria's neighbours cope with the exodus.

Russia and China too contributed to appeals from the International Committee of the Red Cross, and Moscow appeared to have "exercised useful influence" in convincing Damascus to cooperate with the international humanitarian response.

But the EU experts called for greater access to detainees, more visas for foreign aid workers, and increased protection for those working with the Syrian Arab red Crescent to provide medical help.

They said the European Union had prepared contingency plans to evacuate 25,000 to 30,000 EU passport holders if necessary, but that the majority had dual nationality.