Two strong aftershocks measuring 6.5 and 6.1 on the Richter scale rocked Indonesia's northwestern Aceh province on Wednesday, the Meteorology, Climatology and Geophysics Agency (BMKG) said in Jakarta.

Acehnese people try to go to higher ground after a powerfull earthquake hit the western coast of Sumatera in Banda Aceh on April 11, 2012. A massive earthquake struck off Indonesia's Sumatra island, US and Indonesian monitors reported, prompting an Indian Ocean-wide tsunami alert. [Xinhua photo]

Earlier, a massive earthquake measuring 8.5 on the Richter scale jolted off the west coast of northern Sumatra, Indonesia at 15:38 local time (8:38 GMT) and prompted panic among people living in Aceh, West Sumatra and Bengkulu provinces.

According to Metro TV, the strong earthquake led trees and buildings to sway. Hospitals evacuated patients to stay outside.

There have been no reports on collapsed buildings or casualties so far.

The U.S. Geological Survey said that an earthquake measuring 8. 6 on the Richter scale jolted off the west coast of northern Sumatra at 8:38:37 GMT on Wednesday.

The epicenter, with a depth of 22.90 km, was initially determined to be 435 km southwest of Banda Aceh in northernmost Sumatra.

According to Prih Haryadi, an official of BMKG, tsunami warning has been issued in the wake of the major quake. "Based on our model calculation, the tsunami wave may reach 6 meters high," Prih told Metro TV.

The tsunami warning is effective in Aceh only, he said.