The Indonesian meteorology and geophysics agency (BMG) on Tuesday warned of aftershocks following an 8.7-magnitude quake that shook Nias island near Sumatra late Monday.

"There have been 20 aftershocks since the major quake, but their magnitudes were at 4 to 5 on the Richter scale that only local people could feel," BMG official Suharjono said here.

He said the aftershocks are less likely to trigger a tsunami.