The Prime Minister has said that other EU countries must "step up to the plate" and follow Britain's lead by funding refugee camps around Syria.
David Cameron said there was a "direct connection" between shortfalls in aid for camps in Lebanon, Jordan and Turkey and the influx of migrants into Europe.
Speaking during a visit to the Za'atri camp, which houses 90,000 Syrians in Jordan, he said that without funding, "many, many, many" more people would take the decision to risk a perilous Mediterranean crossing.
Cameron met children and young people at the UNICEF Makani Centre at the Za'atri refugee camp near Amman in Jordan, where he saw how the settlement is housing thousands of Syrian families.
Earlier, in Lebanon, he visited the Dalhamiyet Zahle camp, where he met families struggling to survive on on reduced $13.50 a month handouts after the World Food Programme was forced to cut back support.
Cameron has come under increasing pressure to join an EU scheme to relocate 160,000 migrants - many of them Syrian - who have entered Europe in recent months.
But he has argued the scheme would encourage more to risk the perilous Mediterranean crossing, which has claimed many lives.
He said people he spoke to at Za'atri and Dalhamiyet Zahle camps wanted to go home to a peaceful Syria, not to Europe.