The quake occurred as scientists continued to monitor an active volcano near the Polynesian kingdom of Tonga, an archipelago of about 170 islands two-thirds of the way from Hawaii to New Zealand. (Trevor Gregory/Associated Press)Part of the South Pacific has been rocked by a powerful magnitude-7.9 earthquake.

The U.S. Geological Survey said the quake struck Friday morning at a depth of 10 kilometres about 200 kilometres south-southeast of Nuku'Alofa, the capital of the Polynesian kingdom of Tonga, and sparked an initial tsunami warning, which was later cancelled.

People living on Tonga's main island of Tongatapu saw trees sway and felt houses shake, but there were no reports of damage or injury.

New Zealand seismologist Craig Miller said a rolling motion from the quake was reported by residents on the east coast of New Zealand's North Island.

Radio stations in Tonga, an archipelago of about 170 islands two-thirds of the way from Hawaii to New Zealand, broadcast warnings that people should move away from coastal villages because of the tsunami threat.

The quake was reported as scientists continued to monitor a volcano that has been spewing ash and steam for a fifth day in the region.

The eruption is 10 kilometres from the southwest coast of Tongatapu in an area where up to 36 undersea volcanoes are clustered

Tongan government chief seismologist Keleti Mafi said there are fears the quake could lead to more molten lava and ash flowing into the sea.

"The strength of the earthquake could crack the volcano's [undersea] vent and allow more magma to be ejected," Mafi told the Associated Press.

With most of the volcano underwater, much of the volcanic ash has been soaking into the water. Trade winds were blowing the rest of it away from Tongatapuso, officials said.

David Bellwood, a marine biology professor at Australia's James Cook University, said the flowing lava would have little effect on marine life — it will eventually harden and create new land — but a large amount of ash would kill anything in the immediate vicinity.

With files from the Associated Press and Australian Broadcasting Corp.