An earthquake with a magnitude of 5.7 has struck northwest of the South Pacific island nation of Vanuatu, the US Geological Survey says.

The quake today was initially reported with a magnitude of 6.0 but was later downgraded by the USGS.

It was recorded at a depth of 20km and was located around 250km northwest of Vanuatu's Santo island, the USGS said.

There was no immediate tsunami warning or any reports of damage or casualties.

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- AAP