The Baptist Union of Great Britain, the Church of Scotland, the Methodist Church and the United Reformed Church say governments have not done enough so far.
The UN is appealing for 7.7 billion dollars (£5.4 billion) to fund aid operations for the millions of people displaced in Syria, and the neighbouring countries taking them, over the coming year.
It comes after last year's UN appeal for 2.9 billion dollars (£2 billion) was 60% under-funded.
Rt Revd Dr Angus Morrison, Moderator of the General Assembly of the Church of Scotland, who has recently returned from a visit to Egypt where he met with Syrian refugees, said: "To see close up the human catastrophe which is unfolding in Syria and the surrounding region, is harrowing, distressing and deeply humbling.
"We have to do more, meeting immediate need within Syria and the wider region whilst redoubling our efforts for a lasting and just peace."
President of the Methodist Conference, Revd Steve Wild, said: "The bombing and shelling of civilian areas by the Syrian Government and other groups has caused immense death and suffering. The disregard for civilian life has been shocking; it is callous and totally unacceptable even in war."
He also spoke about the situation in the Syrian town of Madaya, which has been cut off from supplies for weeks.
"It is a fundamental principle under international humanitarian law that those fighting a conflict must not restrict the delivery of aid to victims," Revd Wild added.
"The parties to the conflict who are meeting in Geneva must tackle this issue. They have a responsibility to ensure that life-saving aid gets to all in need."
Mr John Ellis, Moderator of the General Assembly of the United Reformed Church said: "We welcome the UK's commitment to aid for Syria, and our Government's initiative in hosting this international pledging conference in London.
"We want every refugee child in Jordan, Lebanon and Turkey to have the opportunity to attend school - but this aim, as outlined by the United Nations, needs funding.
"It is our sincere hope that the international community as a whole will substantially increase its commitment to a situation that is worsening as we speak. Increased funding will mean that those currently affected by the hostilities will not have to suffer even more in the years to come."