An earthquake with a magnitude of 7.3 struck off the west coast of Indonesia's Northern Sumatra on Wednesday, the US Geological Survey reported.

A tsunami warning has been issued by the Pacific Tsunami Warning Center.

The quake, which had a depth of 29 kilometres, hit at 2:37 am (1837 GMT) around 261 miles southwest of Banda Aceh.

An official at Indonesia's Meteorology and Geophysics Agency said it also had put out a tsunami alert, but that there were no immediate reports of casualties.

"The quake was in the sea and a tsunami warning is in force, but so far we have no reports of casualties or damage," Suharjono, the agency's technical chief, told AFP.

The agency initially reported a 7.6-magnitude quake, but later downgraded that to magnitude 7.1.

The Pacific Tsunami Warning Center said the threat of a widespread tsunami did not exist but added: "There is a very small possibility of a local tsunami that could affect coasts located usually no more than a hundred kilometres from the earthquake epicentre."

In Banda Aceh, the ground shook for 30 seconds and terrified residents rushed out of their homes, but they returned indoors shortly after, an AFP reporter there said.

In December 2004, a giant 9.1-magnitude quake off the coast of Aceh province on the northern tip of Sumatra triggered a tsunami in the Indian Ocean that claimed 220,000 victims, with Indonesia accounting for three-quarters of the figure.