The UN secretary general says that half of Syria’s population, some 9.3 million people, are in urgent need of aid.
Ban Ki-moon was speaking at a donor conference in Kuwait promoting the UN’s largest ever appeal for a single issue.
The meeting aimed to raise $6.5bn (£4bn) – it had secured $2.4bn in pledges by the end of Wednesday.
Meanwhile, Syria’s deputy foreign minister has said Western intelligence agencies have held talks with Damascus on combating Islamist groups in Syria.
The UK government denied having any such co-operation with Damascus.
US state department spokeswoman Marie Harf said the minister’s assertion was not true, and that it was “absurd to consider Assad or the regime a partner in countering that threat”.
Islamist groups, often made up of foreign fighters, have become increasingly assertive in Syria, clashing with other rebel movements and causing international concern.
On Wednesday, at least 26 people were killed by a car bomb in the northern city of Jarablus, said the UK-based Syrian National Observatory activist group. It blamed the attack on the al-Qaeda-linked Islamic State in Iraq and the Levant (ISIS) and said most casualties were rival rebels.
Mr Ban told the Kuwait conference the conflict had “set back Syria years, even decades” and that it was “vital for this region and our world that the burden is shared”.
Courtesy BBC World News