JAKARTA : A 5.6-magnitude earthquake struck off Indonesia's Sumatra island on Monday, but there was no tsunami threat and no immediate reports of damage, geologists said.

The quake hit at 5:43 pm (1043 GMT), 43 kilometres (27 miles) northeast of Pagai Utara island in West Sumatra province, the Indonesian meteorology and geophysics agency said via text message.

The quake struck at a depth of 20 kilometres, it said.

"There's no reports of damage so far," agency geologist Apriliyanto told AFP.

No tsunami alert was issued for the quake, he said.

At least 23 people died and thousands of homes were damaged last September when an 8.4-magnitude quake hit West Sumatra and other areas.

Indonesia was the nation worst hit by the earthquake-triggered Asian tsunami in December 2004 that killed 168,000 people in the country's Aceh province.

The Indonesian archipelago sits on the so-called Pacific Ring of Fire, where continental plates meet, causing seismic and volcanic activity. - AFP/ms