A tsunami threat to Fiji has been cancelled, shortly after a powerful earthquake struck off the South Pacific island nation, prompting many panicked coastal residents to evacuate to higher ground, the Pacific Tsunami Warning Centre says.

The quake, which hit at 10.52am (8.52am AEST), was centred 280 km southwest of Fiji's capital, Suva, at a shallow depth of about 15 kilometres, the US Geological Survey said.

It was originally recorded with a magnitude of 7.2 but was later downgraded to 6.9.

There were no initial reports of widespread casualties or damage.