A magnitude 6 earthquake has struck south of Vanuatu, but no tsunami warnings have been issued.

It was downgraded from an initial measurement of magnitude 6.5.

The US Geological Survey gave the quake a green alert rating, meaning there is a low likelihood of casualties and damage.

It said the quake struck at 3.34am (local time), 288km southeast of Isangel, on the island of Tanna. It was 10km deep.

An advisory from the Pacific Tsunami Warning Centre in Honolulu said no action is required "based on earthquake information and historic tsunami records".