SYDNEY (Reuters) - An earthquake with a magnitude of 5.7 struck early on Tuesday about 227 km north-northwest of Santo near the South Pacific nation of Vanuatu, the U.S. Geological Survey said.

The quake struck at a depth of 10 km (6 miles), the USGS said. No tsunami warning was issued by the Pacific Tsunami Warning Center.

The USGS had given a preliminary estimate of the magnitude as 6.1.

(Reporting by Cecile Lefort; Editing by Kevin Liffey)