Germany led the way at a two-day video conference by vowing to contribute 1.74 billion euros ($2 billion), ahead of $600 million from the United States.
"The Syrian tragedy must not last another 10 years. Ending it begins by restoring hope. It begins with our commitments - here, today," German Foreign Minister Heiko Maas said.
The UN warns the need for aid has increased on the back of the coronavirus pandemic and a slump in the value of the Syrian pound.
That is despite the intensity of the fighting dropping inside in Syria after the Russia-backed forces of President Bashar al-Assad have reconquered most of the country.
The UN says that over $10 billion is needed in 2021 -- $4.2 billion for humanitarian relief inside Syria, and the rest for refugees sheltering in the region.
"For 10 years, Syrians have endured death, destruction, displacement and deprivation," United Nations Secretary-General Antonio Guterres said in a video message.
"And things are getting worse, not better. More than 13 million people need humanitarian assistance to survive this year. That's over 20 percent more than last year, and the majority of the population is now facing hunger."
The fifth Brussels Conference on Syria, co-hosted by the European Union and the UN, brings together more than 50 countries and 30 international organisations in the biggest annual drive for pledges to assist those hit by the war.
The total amount of money pledged is set to be announced at 1800 GMT on Tuesday.
Syria's neighbours including Turkey, Lebanon, Jordan and Iraq struggle to cope with the burden of housing most of the millions of refugees who have fled the conflict.
Overall the UN says that 24 million people need support in Syria and across the region - a rise of four million from last year.
Previous pledges
The previous donor conference last June overall raised pledges of $5.5 billion for 2020, according to the United Nations.
The European Commission counted a total of $7.7 billion, with just under a third carried forward to 2021.
The EU and its 27 member states -- which worry that failure to help refugees in the Middle East could see them come to Europe -- provided two-thirds of the funds.
The bloc says overall it has mobilised around $29 billion since 2011.
The war in Syria has killed more than 388,000 people and displaced millions since the regime's brutal repression of anti-government protests a decade ago.
Efforts have stalled to find a lasting peace deal to end a conflict that has pitched world powers against each other and fuelled the rise of the Islamic State group.
US Secretary of State Antony Blinken on Monday launched an impassioned plea at a virtual UN Security Council for key player Russia to reopen aid crossings to allow greater humanitarian access.
Regime ally Moscow last year wielded its veto to oppose the opening of more crossings into Syria on the grounds that they violate the Damascus government's sovereignty.
While Russia has essentially declared Assad the victor of the war, the United States and its European allies are adamant that there must be accountability for crimes over a decade of conflict.
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