The Indonesian island of Lombok has been hit by a strong tremor, as it still reels from the impact of Sunday's earthquake.
The USGS said the latest quake had a magnitude of 5.9 - there are reports it brought down some buildings.
Rescue workers are still digging through rubble and trying to get aid to victims of the earlier, larger quake.
The Red Cross has said that incident, which killed at least 131 people, was "exceptionally destructive".
Some villages had "completely collapsed", said a Red Cross official in Lombok, Christopher Rassi.
The official death toll stands at 131, but local media are reporting figures as high as 347.
The government says more than 1,400 people were injured and more than 156,000 displaced.
The national disaster agency said all those numbers were likely to increase, as more information comes in and more victims are found.
The new tremor hit off the north-west coast of Lombok. Reuters said it sent people running in to the streets in panic. Indonesia's meteorological agency put the magnitude at 6.2. No tsunami warning has been issued.
It is likely to have been the strongest of the more than 340 aftershocks in the area since Sunday.