JAKARTA : A 6.6-magnitude earthquake struck off Indonesia's Sumatra island on Sunday, the US Geological Survey said, but there were no immediate reports of damage.

The earthquake, which hit at 9:46 pm local time (14:46 GMT), occurred at sea about 155 kilometres southwest of Padang in Sumatra, the USGS said.

A tsunami warning was not issued for the quake, which struck at a depth of 35 kilometres.

A worker at the main hospital in Padang, who gave his name as Reno, said the quake was not felt very strongly there.

"There was no need to evacuate patients," he told AFP.

Indonesia's geophysics and meteorology agency measured the quake at 6.5 and put it at a depth of 57 kilometres. It said the quake's epicentre was 151 kilometres northwest of the town of Mukomuko.

The director of Mukomuko hospital said he was at home when the quake rumbled quite strongly for about 15 seconds.

"Everybody rushed out of their homes," he told AFP, adding that he had rushed to the hospital in case of casualties. But he said on his way that he saw no signs of damage and people had returned inside again.

The Indonesian archipelago sits on the Pacific Ring of Fire, where continental plates collide and cause frequent seismic and volcanic activity.

Indonesia was the nation worst hit by the earthquake-triggered Indian Ocean tsunami in December 2004, which killed some 168,000 people in Aceh province lying at the northern tip of Sumatra island. - AFP/de