The United Nations - along with international aid agencies and non-governmental organisations (NGOs) - is expressing scepticism over the eventual delivery of the hefty US$2.5 billion in pledges made by donors for tsunami disaster relief operations in south and southeast Asia.

Asked if governments could falter on their pledges, UN Secretary-General Kofi Annan told reporters Monday: "If we go by past history, yes, I do have concerns."

"We have got lots of pledges (totalling) over US$2.5 billion but it is quite likely that, at the end of the day, we will not receive all of it," he added.