The United Nations needs another $US3.5 billion to deal with the Syrian refugee crisis up to the end of this year.
Humanitarian affairs chief Valerie Amos said on Friday that donor countries should look again at their contributions and be as generous as they can.
She said the crisis and the huge movement of people have far-reaching implications for global peace and security.
The UN estimates two million people have fled Syria.
British Prime Minister David Cameron on Friday announced an additional £52 million ($80m) in aid for Syria - much of it for medical training and equipment to help civilians targeted by chemical attacks.