UN Secretary-General Ban Ki-moon has appealed for an end to the violence in Syria and urged more aid to address a situation he said was catastrophic and worsening by the day.
He denounced the "unrelenting horrors" in the war, and said much remains to be done in securing humanitarian aid.
"How many more people will be killed if the current situation continues?," Mr Ban said, speaking at a conference of donors in Kuwait.
Donors have pledged over $1bn in aid for Syrians displaced by nearly two years of fighting.
Minister of State for Cabinet Affairs Sheikh Mohammad Abdullah al-Mubarak al-Sabah told reporters he hoped the total pledges would reach the target of $1.5bn (€1.1bn) set by the UN by the end of the meeting later in the day.
The United Nations has said over 60,000 people have been killed in the 22 months since the conflict began.
Mr 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".
Four million Syrians inside the country need food, shelter and other aid and over 700,000 more have escaped to neighbouring countries since the conflict began, according to the UN.
Jordan's King Abdullah told the gathering that Syrians had taken refuge in his country in their hundreds of thousands but Jordan's ability to help was at its limits.
"We have reached the end of the line, we have exhausted our resources," he said.
UN humanitarian chief Valerie Amos said farming was in crisis, hospitals and ambulances had been damaged and even painkillers were unavailable.
Harsh winter weather had made matters worse, and people were without winter clothes, blankets and fuel.
Women and children were particularly at risk, she said.
She added: "We are watching a human tragedy unfold before our eyes."
The aid would fund operations for the first half of this year, but the UN has so far received pledges covering just 18% of the target, unveiled last month as the scale of Syria's humanitarian crisis escalated sharply.
Even if pledges are made, aid groups have found in the past that converting promises into hard cash can take time.
Mr Ban said much more remained to be done.
"The situation in Syria is catastrophic and getting worse every day," he said. "Every day Syrians face unrelenting horrors," he said, adding this included sexual violence and detentions.
Aid officials hope the fact that the conference is being held in Kuwait will encourage other wealthy Gulf Arab states, who have led regional opposition to President Bashar al-Assad, to support the international aid effort.
Many Gulf states have sent assistance, but aid workers say their efforts have been haphazard and rarely coordinated with other aid agencies, hampering their ability to plan a sustained relief programme.
Syria's main opposition coalition has criticised the UN appeal and its arrangements for distributing aid inside Syria, saying the organisation has effectively ceded control to the Syrian government and failed to deliver all but a bare minimum of aid to areas controlled by Mr Assad's opponents.
Israeli jets attack convoy on Syrian-Lebanese border
Israeli forces attacked a convoy on the Syrian-Lebanese border overnight, a Western diplomat and regional security sources said.
The sources, four in total, all of whom declined to be named because of the sensitivity of the issue, had no further information about what the vehicles may have been carrying, what forces were used or where precisely the attack happened.
In the run-up to the raid, Israeli officials have been warning very publicly of a threat of high-tech anti-aircraft and anti-tank missiles reaching Israel's enemies in the Lebanese militant group Hezbollah from Syria.
They have also echoed US concerns about Syria's presumed chemical weapons arsenal.
The Lebanese army reported a heavy presence of Israeli jets over its territory throughout the night.
Syria 'breaking up before everyone's eyes'
Meanwhile, UN-Arab League mediator Lakhdar Brahimi has warned the UN Security Council that Syria is "breaking up before everyone's eyes," diplomats said.
He said that Mr Assad may be able to cling to power for now but that the regime's legitimacy has probably been "irreparably" damaged.
Mr Brahimi appealed to the 15-nation council to overcome its deadlock and take action to help put an end to the Syrian civil war.
However, it was not clear whether his latest report, which diplomats said was his bleakest since his appointment last year, would persuade Russia to agree to support concrete UN steps to try to halt the bloodshed.
Mr Brahimi suggested that attempts to end the conflict had not progressed in the past two months.
He said it was up to the Security Council to end its impasse.
He said the principles of a political transition in Syria, agreed to at talks among major world and regional powers in Geneva in June last year, could form the basis for a Security Council plan of action.
Russia has said that insisting on Mr Assad's departure as a condition for peace negotiations between the government and the opposition forces would prevent such talks from ever taking place.
The opposition, backed by the United States and much of Europe, has made plain that Mr Assad can play no role in a future Syrian government.