WELLINGTON (AFP) - A powerful 6.8-magnitude earthquake struck off Fiji on Sunday shortly after another hit off Tonga but there was no threat of a tsunami, US seismologists said.

The offshore quake occurred at 9:28 am (2128 GMT Saturday), centered around 100 kilometres (62 miles) northeast of the Fijian island of Ndoi at a depth of 594 km, the US Geological Survey (USGS) said on its website.

It came 20 minutes after a 6.3-magnitude quake hit 124 kilometres northwest of Tonga.

The Hawaii-based Pacific Tsunami Warning Center said neither quake posed a tsunami threat.

In its preliminary impact assessment, the USGS said there was a low likelihood of casualties and damage.