Banda Aceh – The United Nations on Tuesday pronounced the often chaotic relief effort to aid Indonesian tsunami victims a success but warned that the toughest part of the operation was still to come.

"The peak of the emergency operation is behind us," said Joel Boutroue, the UN's deputy coordinator in Aceh province, which was home to most of Indonesia's estimated quarter of a million disaster dead.

"I would like to say, all in all, in hindsight it has been a fairly successful operation, thanks in great part to the government of Indonesia."

But with the US and Australian militaries – key components in the emergency relief operation – scaling down their presence, Boutroue said the hardest work lies ahead as the humanitarian effort moved into the long-term.

"Without being facetious, I would say that in a way it was the most easy part and the difficult part starts now. We're going into a transition phase where we need to be more focused," he told reporters.

Boutroue's positive assessment came in contrast to earlier reports of poor coordination, which led to bottlenecks of aid supplies building up at airports as thousands of starving people were stranded on remote stretches of coastline.

Problems could be clearly seen last week. Mountains of soaking clothes were left lying on beaches along Aceh's devastated west coast, with no transport to take them to those in need.

The WHO's director of health action in crises, David Nabarro, last week issued a scathing assessment of relief operations, saying the UN could have moved more quickly to prevent delays in delivering aid to those in need.