Nearly 12 million people inside Syria need emergency aid, and 5.6 million refugees are being housed and fed in Turkey, Jordan, Lebanon, Iraq and Egypt.
The European Union, the world's biggest aid donor, pledged 2 billion euros ($2.26 billion) for this year, a sum which includes money already agreed for Syrian refugees in Turkey under a deal with Ankara to take in Syrians.
U.N. High Commissioner for Refugees Filippo Grandi, who recently visited Syria, told the conference that Syrians had lived through horrors "almost beyond contemplation" and urged countries to help finance food, water and medicine supplies.
Donors must contend with U.S. President Donald Trump's demand that allies carry more of the burden. His government last year failed to submit a pledge, although U.S. funding commitments eventually came in, EU diplomats said.
The United States pledged more than $397 million on Thursday, although that was less than the $697 million offered in 2017, according to the U.S. State Department. Data for 2018 was not immediately available.
Mark Lowcock, U.N. under secretary general for humanitarian affairs, said that emergency aid would not solve the Syria crisis. "It requires a political solution," he said.
That underscored Europe's dilemma in its efforts to isolate Assad. The EU has repeatedly made longer-term reconstruction support conditional on a U.N.-led peace process to end a war that has killed hundreds of thousands of people.
But with the U.N. process stuck, Russia's 2015 military intervention proving decisive for Assad, and Arab neighbours considering re-establishing diplomatic ties, European governments are divided over whether to rebuild.
Germany, France, Britain and the Netherlands are forthright in defending a policy of withholding reconstruction money until a transition begins that would lead to Assad leaving power.
However, if that were not to happen, European diplomats say they would see it as Russia's responsibility to seek a solution, given its outsized military role and support for Assad.
"The road to stability runs through Moscow," a European diplomat said.
The EU has imposed an oil embargo on Syria as well as export restrictions, a freeze on central bank assets and hit 270 people and 72 entities with asset freezes and travel bans. Italy, Austria and Hungary, all critics of European immigration policy and closer to Moscow, favour talking to Syrian authorities to allow millions of refugees to go home.
The European Commission has begun to put some money aside to help returning refugees, while the EU's foreign service is using satellite images to study potential areas of reconstruction, diplomats say. ($1 = 0.8850 euros)
(Additional reporting by Lesley Wroughton in Washington and Gabriela Baczynska in Brussels; Editing by Alison Williams) ((firstname.lastname@example.org; +32 2 287 6835; Reuters Messaging: email@example.com))