"What we hope is now that the international community will really speed up work on the early-warning system for the tsunami," Under-Secretary-General Jan Egeland told a news briefing in New York, noting that there were unconfirmed reports of deaths on one island near the epicentre and accounts of limited damage in the region, which suffered most from the earlier disaster.

"A little bit stronger earthquake this time [and] we could have another major tsunami in the middle of the night," he added, referring to the giant waves of 26 December that killed more than 200,000 people and left up to 5 million in need of basic services in a dozen Indian Ocean countries. "We need that early warning system up and going."

But the UN, which mounted its largest ever coordinated relief effort following the December catastrophe, already has relief workers in place this time because of the ongoing reconstruction.

"We have helicopters, we have trucks, there are now more than 1,000 international relief workers in Sumatra, [and] 300 international relief organizations present in Sumatra," Mr. Egeland said, noting that a helicopter survey will start at dawn for those islands which are feared to have suffered most within the 50-kilometre zone of the epicentre and are home to some 10,000 people.

"So we're in a very different situation than we were on the 26 December, where we had no possibility, no permission to go outside Banda Aceh (in northwest Sumatra) since Aceh was a conflict zone," he added.

"My impression is that the system worked far better this time, that there was a vigilance. Not only did we have surveillance and information to the countries but we also had governments reporting out to the local authorities. We will have to assess that tomorrow, how well it worked in the various locations," he said.

As soon as news of the quake came in, the UN Office for the Coordination of Humanitarian Affairs (OCHA) contacted immediately all UN offices in the dozen countries affected by the tsunami. In Banda Aceh, the local authorities did not give the order to evacuate but some people fled on their own, Mr. Egeland said.

"We see this as yet another example of panic spreading, of deep concern in an already mentally scarred population. It will cast shadows not only over our relief effort but also the reconstruction and rebuilding efforts," he added. "Now the big fear again rises. Is it too early? Can we start rebuilding?"

He appealed to governments to honour the billions of dollars they have already pledged for the reconstruction and development phase of the earlier disaster.