A powerful and shallow earthquake struck Wednesday off Indonesia's Sumatra island, sending panicked residents rushing from their homes in a region hit hard by quakes and tsunamis in the past.

Local authorities initially issued a tsunami warning but lifted it several hours later, and there were no immediate reports of casualties or damage.

The 7.8 magnitude undersea quake struck at a depth of 24 kilometres (15 miles) at about 6:50 pm (1250 GMT), the US Geological Survey said.

The epicentre was several hundred kilometres from the Mentawai Islands, a small chain southwest of the country's main western island of Sumatra.

The quake was felt strongly in Padang, a major city on western Sumatra, sparking panic. An AFP journalist there said people ran from their homes and fled to higher ground by motorbike, car or on foot.

Traffic ground to a halt and there was a sense of panic on the streets, the journalist said.

The local BMKG quake-monitoring agency issued a tsunami alert for several provinces with coastlines on western Sumatra but later lifted the warning.

The small Mentawai archipelago is regularly hit by quakes and in 2010 was devastated by a quake-triggered tsunami that left hundreds dead.

Aceh province on the tip of Sumatra was devastated by a quake-triggered tsunami in 2004. The tsunami killed more than 170,000 people in Indonesia and tens of thousands more in other countries around the Indian Ocean.

- Warning sirens -

Disaster agency spokesman Sutopo Purwo Nugroho said tsunami warning sirens were after the earthquake activated and "people responded to the early warning by evacuating to higher places".

Bambang Soelistyo, head of Indonesia's search and rescue agency, said there had not been any reports of casualties so far.

"In the Mentawai Islands, there has been no reports of damage and people have been evacuating to higher areas," he told AFP.

Indonesia sits on the Pacific "Ring of Fire", where the meeting of continental plates causes strong seismic activity, and is frequently hit by earthquakes.

Hundreds died when a major quake struck near Padang in 2009.

Plans for evacuation shelters and improved roads to provide better escape routes from tsunamis since 2004 have mostly not been realised, according to experts.

In Australia, to the east of the Indonesian archipelago, authorities said there was no tsunami threat to the Australian mainland but issued a marine threat to Cocos Island and Christmas Island off the east coast.

Evacuations were not required, according to the Joint Australian Tsunami Warning Centre, but there was a possibility of dangerous waves and strong currents.

People in Christmas Island were advised to get out of the water and move away from the immediate water's edge.

Sri Lanka's Disaster Management Centre, a government agency for the Indian Ocean island where tens of thousands died in the 2004 disaster, did not issue a tsunami warning Wednesday, but said it was monitoring the situation.

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