The Pacific Tsunami Warning Center (PTWC) said a tsunami had been generated but its likely impact was not yet clear.
It advised national authorities across the Indian Ocean region to "take appropriate action".
The region is regularly hit by earthquakes. The Indian Ocean tsunami of 2004 killed 170,000 people in Aceh.
One official quoted by Reuters said a 17cm (6.7in) tsunami had been generated and was heading for the coast of Aceh.
The US Geological Survey (USGS), which documents quakes worldwide, said the Aceh quake was centred 33km (20 miles) under the sea about 495km from Banda Aceh, the provincial capital.
It was initially reported as 8.9 magnitude but was later revised down to 8.6 by the USGS. Strong aftershocks were also reported.
The BBC's Karishma Vaswani in Jakarta says there were reports of the ground shaking for up to five minutes. (+ video) » | Wednesday, April 11, 2012
THE NEW YORK TIMES: Indonesia Sets New Tsunami Warning as Aftershock Hits: BANGKOK — A powerful 8.6-magnitude earthquake struck more than 200 miles off the coast of the Indonesian island of Sumatra on Wednesday, spreading panic among residents and reviving memories of the devastating 2004 earthquake and tsunami in the same area. » | Thomas Fuller | Wednesday, April 11, 2012
AL JAZEERA ENGLISH: Indian Ocean on tsunami alert after quakes: Indian Ocean tsunami warning extended after strong aftershock off northern Indonesia's coast. » | Source: Al Jazeera and agencies | Wednesday, April 11, 2012
