The three party leaders have not said how much more will be pledged on top of the £523 million already donated by the UK but they have given full support to the United Nations' £4 billion appeal to help the millions forced to flee the civil war.
They said it was essential that aid reached the refugees as the fate of an entire generation of Syrian children "hangs in the balance" and urged other countries to follow Britain's lead.
"More than nine million people in Syria are suffering this Christmas in desperate need of our help. Four million of them are children; the equivalent of every primary school pupil in England. This is an urgent humanitarian issue that transcends the differences of party politics," they said.
"Our country has a long history of helping those in dire need. As a nation, we have already given £523m, including many generous donations from members of the public. But even more is needed to cope with the scale of the suffering.
"The UN has recently launched a new call for £4bn; the largest appeal in its history. Britain will give more and we urge other nations to do the same."
The party leaders said that, at the same time, it was vital all sides in the conflict guaranteed humanitarian access to ensure help reached all those in need.
"Too many children are out of the reach of agencies like Unicef. Britain helped broker the agreement at the UN Security Council for unhindered access. Now it is essential to ensure this is delivered with secure routes to get the life-saving aid to all those who need it."