International Development Secretary Justine Greening says the world has a "historic opportunity" to draw up a new deal to help Syrian refugees fleeing their country's brutal civil war.
Aid agencies are gathering in London ahead of a major international funding conference on Thursday hosted by Britain, Germany, Norway and the United Nations.
The UN is appealing for 7.7 billion US dollars (£5.4bn) to fund aid operations for the millions of people displaced in Syria and the neighbouring countries over the coming year.
It comes after last year's UN appeal for 2.9 billion US dollars (£2bn) was 60% under-funded.
Addressing a Syrian civil society conference at the Royal Society in central London, Ms Greening said tomorrow's conference is "a critical moment for us to step up our efforts and help those affected by the Syria crisis".
She said that the UK has pledged more than £1bn in aid - its biggest ever humanitarian response to a crisis.
But she said more needs to be down and called for the international community to come together to scale up their aid effort.
Ms Greening said: "What we have done so far just isn't enough. So this conference can and must galvanise significant new funding from around the world to not just meet the immediate needs of people caught up in this crisis, but to meet the longer term needs as well.
"Tomorrow we have got to see an unprecedented response to an unprecedented crisis."
She said the time had come for "the world to step up" and that tomorrow's conference "is a historic opportunity to galvanise the funding".
She added: "I truly believe that tomorrow the world can offer an alternative vision of hope for the Syrian people."
US president Barack Obama spoke on Tuesday by phone to David Cameron, and told the Prime Minister that the US will soon announce "significant new contributions" to assist Syria's humanitarian crisis.
The White House said US Secretary of State John Kerry would announce the aid at the summit.
Both leaders are calling for better access for humanitarian aid to reach Syria.
A coalition of more than 90 aid agencies and humanitarian organisations - including Oxfam and Amnesty International - is pressing for governments to go further with a new, multi-year funding plan.
They also calling for measures to enable neighbouring countries - such as Turkey, Jordan and Lebanon - to allow refugees to work and to ensure they have access to essential services such as healthcare and children's education.
Former foreign secretary David Miliband, who now heads the International Rescue Committee, said: "Syrians are facing a war without law and a war without end.
"The latest harrowing scenes from the besieged town of Madaya and the rising pressure on neighbouring states need to spur political leaders to act.
"The relentless suffering of the Syrian people should be a global call to action for humanitarian assistance to alleviate suffering and for political action to bring the war to an end."
The meeting comes as King Abdullah of Jordan - which has seen an influx of hundreds of thousands of refugees - warned his country could no longer cope with the pressures of accepting so many people.
"Sooner or later, I think, the dam is going to burst," he warned in a BBC interview. "The psyche of the Jordanian people, I think it's gotten to boiling point."