A 7.4-magnitude earthquake struck 255 kilometres (158 miles) southwest of Tonga early Friday, according to the US Geological Survey.

The under-sea quake struck at 5:19 am (1719 GMT Thursday) and sparked a tsunami alert, although no damage was expected due to its depth of 171 kilometres, according to the Pacific Tsunami Warning Center.

The earthquake, centred 282 kilometres southwest of Tonga's capital Nuku'alofa, was followed four hours later by a second powerful 6.6 tremor 84 kilometres northwest of the town.

The second quake, at a depth of 103 kilometres, also generated a tsunami warning.

"A destructive tsunami was not generated based on earthquake and historical tsunami data," the centre said of each quake.

The two quakes come 12 days after a similar 6.5-magnitude earthquake struck the area, at a depth of 205 kilometres.

Tonga, almost 2,000 kilometres (1,240 miles) northeast of New Zealand, lies on the so-called Pacific "Ring of Fire", where continental plates collide causing frequent seismic activity.