Donor countries have pledged more than $1.5 billion to aid Syrians stricken by civil war, U.N. Secretary-General Ban Ki-moon said on Wednesday after warning that the conflict had wrought a catastrophic humanitarian crisis.

In a pointed message for Syria's leader, Ban told a fund-raising conference in Kuwait that President Bashar Assad bore primary responsibility to stop his country's suffering after nearly two years of conflict that have cost an estimated 60,000 lives.

"Every day Syrians face unrelenting horrors," Ban told the gathering, adding these included sexual violence and arbitrary killings. Sixty-five people were shot dead execution-style in Aleppo on Tuesday, opposition activists said.

"We cannot go on like this.... He should listen to the voices and cries of so many people," Ban said.

"I appeal to all sides and particularly the Syrian government to stop the killing ... in the name of humanity, stop the killing, stop the violence."

Ban said the one-day conference had exceeded the target of $1.5 billion in pledges. About $1 billion is earmarked for Syria's neighbors hosting refugees and $500 million for humanitarian aid to Syrians displaced inside the country.

The $500 million would be channeled through U.N. partner agencies in Syria and the entire aid pledge would cover the next six months, Ban said.

But in the Syrian capital Damascus, the thud of artillery drowned out any optimism on the streets. Asked about the aid promises, Damascenes were uninterested or despairing.

"Where's the money going to go to? How does anyone know where it's going? It all seems like talk," said Faten, a grandmother from a middle-class family in the capital.

Another middle-class Damascene, a woman in her 70s who asked not to be named, said the money would not make it to Syrians.

"Tomorrow all that money will get stolen. (The middlemen) steal everything. If they could steal people's souls, they would. I wouldn't count on the money," she said.

The oil-rich Gulf Arab states of Kuwait, Saudi Arabia and the United Arab Emirates each promised $300 million at the meeting. Its 60 participants included Lebanon, Jordan, Iran, Tunisia, the United States, Canada, Russia, China, Japan, South Korea, Turkey and a number of European countries.

But relief groups say that converting promises into hard cash can take much time, and one of them said on Tuesday that aid now reaching Syria was not being distributed fairly, with almost all of it going to government-controlled areas.

Four million Syrians inside the country need food, shelter and other aid in the midst of a freezing winter, and more than 700,000 more are estimated to have fled to countries nearby.

More than 60,000 people have been killed in all, according to a U.N. estimate, since the conflict began as a peaceful movement for democratic reform and escalated into an armed rebellion after Assad tried to crush the unrest by force.

Rahmed Hagagy, Sami Aboudi, Mahmoud Habboush and Mirna Sleiman contributed to this Reuters report.