Visiting UN humanitarian chief Tom Fletcher called on Wednesday for a massive aid boost for Syria to respond to "this moment of hope" after the ouster of longtime regime leader Bashar al-Assad.
"Across the country, the needs are huge. Seven in 10 people are needing support right now," Fletcher told AFP in a telephone interview as he visited Syria.
"I want to scale up massively international support, but that now depends on donors. The Syria fund has been historically, shamefully underfunded and now there is this opportunity," he said.
"The Syrian people are trying to come home when it's safe to do so, to rebuild their country, to rebuild their communities and their lives.
"We have to get behind them and to respond to this moment of hope. And if we don't do that quickly, then I fear that this window will close."
Half of Syria's population were forced from their homes during nearly 14 years of civil war, with millions finding refuge abroad.
UN officials have said a $4 billion appeal for Syria aid is less than a third funded.
"There are massive humanitarian needs... water, food, shelter... There are needs in terms of government services, health, education, and then there are longer term rebuilding needs, development needs," Fletcher said.
"We've got to be ambitious in our ask of donors."
"The Syrian people demand that we deliver, and they're right to demand that we deliver," he said. "The world hasn't delivered for the Syrian people for more than a decade."
As part of his visit, Fletcher met representatives of Hayat Tahrir al-Sham (HTS), the Islamist rebel group which spearheaded the offensive that toppled Assad, including its leader Ahmed al-Sharaa and interim prime minister Mohammad al-Bashir.
Fletcher said he received "the strongest possible reassurances" from Syria's new administration that aid workers would have the necessary access on the ground.
"We need unhindered, unfettered access to the people that we're here to serve. We need the crossings open so we can get massive amounts of aid through. We need to ensure that humanitarian workers can go where they need to go without restriction, with protection," he said.
"I received the strongest possible reassurances from the top of that caretaker administration that they will give us that support that we need. Let's test that now in the period ahead."
The Assad regime had long imposed restrictions on humanitarian organisations and on aid distribution in areas of the country outside its control.
Fletcher said that the coming period would be "a test for the UN, which hasn't been able to deliver what we wanted to over a decade now... Can we scale up? Can we gain people's trust?
"But it's also a test for the new administration," he added. "Can they guarantee us a more permissive environment than we had under the Assad regime?
"I believe that we can work in that partnership, but it's a huge test for all of us."