A moderate earthquake struck off the south Pacific island state of Samoa on Saturday, seismologists said, but there was no tsunami alert and no immediate reports of damage.

The US Geological Survey said the 5.5-magnitude quake struck at 2204 GMT (0904 AEDT) around 281(2+8+1=11) kilometres south southwest of the capital Apia.

It struck at a depth of 35km.