The quake, which was on land at a depth of 10 km (6.21 miles), was felt as far away as Singapore and Malaysia and strongly in the nearby cities of Padang in West Sumatra province and in Pekanbaru in Riau Province, witnesses said.

"We continue to monitor and advise people to remain on alert," Dwikorita Karnawati, the head of BMKG, told MetroTV.

"Because this is on land, and the scale is above 6, we are concerned that it could cause some damage."

In Pasaman Barat, about 17 km from the epicentre, a witness told MetroTV patients in a hospital were evacuated from the building.

Indonesia's disaster mitigation agency could not immediately be reached for comment about damage.

Alim Bazar, head of the disaster mitigation agency of Pasaman city near the quake's epicentre, told AFP some buildings suffered cracks.

"The mayor called and ordered all second and third floors in every building should be vacated," he said.

Bazar said there were reports of some injuries, but did not have any details.

Irpanda, a resident of Pasaman city who like many Indonesians goes by one name, told Metro TV that he felt both the first and second tremors.

"At first, the quake only lasted for a few seconds. People fled their homes and buildings nearby were swaying," he said.

"But then another quake happened and it was so strong. More people fled their houses," he said, adding patients at a local hospital were moved outside.

Tremors were also felt in Singapore, witnesses and police said.

"Earth tremors were... felt in certain parts of Singapore at about 9:45 am," the police said in a statement.

The police and emergency services "have received several calls from the public reporting these tremors", the statement added.

Malaysia's meteorological department said in a tweet that "vibrations" were felt on the peninsula's western states.

"Vibrations felt in the western Peninsular area especially in Selangor, Perak, Negeri Sembilan, Malacca and Johor," the department said in the tweet.

Indonesia suffers frequent earthquakes, straddling the so-called "Pacific Ring of Fire", a highly seismically active zone, where different plates on the earth’s crust meets.

Last month, a 6.6-magnitude earthquake struck off Java island, prompting some residents in the capital Jakarta to flee from buildings in panic.

A more powerful 7.4 magnitude in Flores Sea triggered a tsunami warning in December, though caused only minor damage.