A 7.9-magnitude shallow earthquake struck off Indonesia's western island of Sumatra Wednesday, the US Geological Survey said, with local authorities issuing a tsunami warning.

The undersea quake struck southwest of Sumatra at a depth of 10 kilometres (six miles) at about 6:50 pm (1250 GMT), several hundred kilometres from the city of Padang, the USGS said.

The local BMKG quake-monitoring agency issued a tsunami alert for parts of Sumatra, including West Sumatra, North Sumatra and Aceh.

The quake was felt strongly in Padang in West Sumatra for a few seconds, a AFP journalist in the city said. People ran out of their homes to higher ground.

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

However the USGS said there was a "low likelihood of casualties and damage".

"There are likely to be no affected structures in this region," it added on its website.

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