Hundreds of people are believed to have died in Indonesia after a massive earthquake that triggered tsunami alerts around the Indian Ocean.
People panicked and fled within seconds of the quake
It was felt as far away as Malaysia and Singapore, and thousands fled their homes across a region still reeling from December's devastating tsunami.
But three hours after the quake, tsunami alerts were called off, and people began returning home.
The island of Nias off Sumatra was worst hit in the 8.7 magnitude quake.
Aid workers from UK charity Oxfam who have reached Gunung Sitoli, the island's main town, report that roads have collapsed and water supplies have failed, leaving 20,000 people without water.
The 26 December tsunami killed an estimated 300,000 people in a dozen countries - two-thirds of them in Indonesia, mostly in Aceh province.
But many areas hit hard in December did not suffer much on Monday - largely because in places like Banda Aceh, there are hardly any buildings standing.
The islands of Nias and Simeulue off the western coast of Sumatra bore the brunt of the quake which struck at around 2315 local time (1615 GMT) and lasted up to three minutes.
Reports quoting Indonesian officials suggest at least 330 people were killed.
But Indonesia's Vice-President Jusuf Kalla told the BBC "more than 1,000 people were possibly dead" in Nias alone.
Around 80% of buildings had been affected in Gunung Sitoli, Mr Kalla said.
Binahati Bahea, district chief of Gunung Sitoli, told the BBC the number of dead "will rise, because we have not managed to evacuate many buildings and there are many dead people there".
"Our problem at the moment is transportation because many bridges are damaged and it's difficult to reach many towns," he said.
Oxfam staff who went to a mosque in the town being used as a morgue and temporary shelter said 34 bodies had already been brought there.
Alessandra Villas-Boas, a member of the Oxfam team, said: "The water system has failed completely and huge holes have been made in the roads. Bodies are being pulled from the rubble as I speak."
United Nations teams are also in the area. President Susilo Bambang Yudhoyono has declared a state of emergency and is to travel to Nias on Wednesday.
Despite Nias's remoteness, it has become a popular resort in recent years with surfers and the money raised from tourism funded a mini building boom, says the BBC's Tim Johnston in Jakarta.
Reports say it was many of these buildings, made from concrete rather than more flexible traditional materials, that collapsed.
In Banda Aceh, there was pandemonium as people scrambled to higher ground after the government issued a tsunami warning. But further south, in Lamno, there were no authorities present as people fled.
In the Malaysian capital Kuala Lumpur, high-rise buildings were evacuated and people ran into the streets.
Thailand, India, and Sri Lanka - all badly hit by the December disaster - temporarily issued tsunami alerts.
The epicentres of the two earthquakes were about 160km (100 miles) apart.
However no tsunami materialised on this occasion as a much smaller area of the ocean floor moved, seismologists say.