Angry refugees confronted US Secretary of State John Kerry during his landmark visit to the Zaatari camp in Jordan on Thursday, AFP reported.
The refugees demanded that Washington do more to end the war in Syria, and urged a no-fly zone to protect areas along the border and those held by rebel forces.
Kerry first overflew the camp by helicopter, surveying thousands of tents and trailers lined up on the desert sand about 20 kilometres (12 miles) from the Syrian border, in what has now become the fifth largest city in Jordan.
"The stories that I've just heard and the people I've just met put a real face on the level of the humanitarian crisis," the US secretary of state said.
Kerry met six refugees and was briefed by the UNHCR camp manager Kilian Kleinschmidt, who told of the battle to provide for a daily tide of humanity crossing the border from Syria into Jordan.
"The stories are obviously horrendous. The life is very, very difficult," Kerry said, after he became the highest-ranking member of the US administration to visit the camp, home to some 115,000 refugees.
The refugees voiced anger and frustration, repeatedly asking Kerry to press Washington to establish buffer and no-fly zones in Syria.
"Where is the international community? What are you waiting for? We hope that you will not go back to the United States before you find a solution to the crisis. At least impose a no-fly zone or an embargo," said one refugee woman.
"I think the US as a superpower can change the equation in Syria in thirty minutes after you return to Washington," she added.
A grim-faced Kerry replied: "A lot of different options are under consideration. I wish it was very simple. As you know, we've been fighting two wars for 12 years.
"We are trying to help in various ways, including helping Syrian opposition fighters have weapons. We are doing new things. There is consideration of buffer zones and other things, but it is not as simple as it sounds."
But the same woman threatened that once the holy month of Ramadan was over: "We will return to Syria and we will fight with knives. You as the US government look to Israel with respect. Cannot you do the same with the children of Syria?"
Kerry assured the group that "You are not abandoned. We are very aware of how terrible conditions are inside Syria.
"I promise you I will take your voices and concerns back with me to Washington."
The United States is the largest single donor to Syrian refugees and has already pledged some $815 million in humanitarian assistance.
US considers Syria intervention
Kerry's visit comes on the same day as U.S. Joint Chiefs of Staff Chairman Martin Dempsey informed a Senate panel that the Obama administration is deliberating the possibility of using military force in Syria.
The use of “kinetic strikes,” a military term that usually refers to missiles and bombs, “is under deliberation inside of our agencies of government,” he said.
Dempsey voiced his support for lending US support to "moderate" elements of the Syrian opposition, but stressed that the decision to intervene more directly had to be made by "elected officials."
From the air, the Za'atari camp, which opened only a year ago and costs $1 million a day to run, clearly dwarfed the neighbouring villages.
There are no trees or bushes to provide shade from the baking Jordanian sun in an area which before last year was just empty scorpion- and snake-infested scrubland.
Now there are football pitches and a children's playground aimed at keeping some 60,000 youngsters busy.
"The conflict has reached a level of brutality that is indescribable," Kleinschmidt told Kerry.
"The stories are horrible," he said, putting the number of refugees at the seven-square-kilometre (2.8-square-mile) camp at 115,000 -- 70 percent of them children and women.
"I'm managing a temporary city that is beginning to settle," Kleinschmidt said, adding there are three hospitals in the camp as well as schools but only 5,000 out of an eligible 30,000 children attend classes.
Every day some 12 to 15 babies are born at Zaatari.
Despite the camp's air of permanency, Jordanian Foreign Minister Nasser Judeh insisted it was temporary.
"We said when we opened this camp that we look forward to the day we will celebrate closing it. That means these proud Syrian people going back to their homes and living in security and peace and stability," he told reporters.
"The Jordanians themselves even before the Syrian refugee crisis were coping with a difficult set of economic circumstances," said a senior State Department official.
The Arab kingdom says it is playing host to some 550,000 refugees from Syria in camps and in urban areas, but the UNHCR said the latest figures were about 600,000.
The influx of so many people is straining already scant resources such as water and energy and tearing at the nation's social fabric.
The United Nations now estimates 1.8 million people have fled the Syria into neighbouring countries, and that 6,000 a day are leaving.