"My best guess is that by the end of the day we will have heard pledges for 2018 of $4.4 billion", Mark Lowcock, the head of UNOCHA, told a news conference.
Global donors on Wednesday pledged $4.4 billion in aid for 2018 for civilians caught up in the Syrian civil war - well short of what the United Nations says is needed for humanitarian work in Syria and neighbouring countries.
He added that pledges of a further $3.3 billion for 2019 and after were expected at the conference, attended by more than 80 countries, aid groups and agencies.
Staffan de Mistura, the UN's special envoy to Syria, warned on Tuesday of a looming new crisis among the 2.5 population of Idlib in the country's northwest, following the fall of eastern Ghouta to regime forces.
The participants of the Brussels II Conference on "Supporting the future of Syria and the region" have reaffirmed their commitment to Syria's territorial integrity, sovereignty and unity, the Council of the European Union said in a statement late on Wednesday.
"I want particularly to thank the EU, Germany and the United Kingdom who have made exceptionally large pledges today", Lowcock said.
The money raised so far is significantly lower than the sums donated at a similar conference previous year.
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More than 700,000 people have been displaced since the start of this year alone as Assad has stepped up his offensive against rebel forces, intensifying the humanitarian crisis.
He stressed on importance of finding a political solution to the end the eight-year-old conflict and said that the suffering of the Syrian people was aggravated by use of chemical weapons in violation of all worldwide laws and human principles.
The prolonged war in Syria is resulting in donor fatigue among countries struggling to provide humanitarian aid to people displaced by the fighting.
European Union foreign policy chief Federica Mogherini has called on Russia, Iran, and Turkey to do more to scale down the fighting in Syria.
The EU and a number of other countries have refused to rebuild to Syria until the start of meaningful peace moves to end the conflict. Syrian representatives "should have been invited because we are talking about addressing the needs of the Syrian people, so there should be somebody representing the Syrian people", Chizhov added.
About 70,000 people are in dire need of aid in Douma after one of the most intense military battles in Syria's seven-year war.