Relief workers scrambled to reach quake-ravaged villages in the Solomon Islands Saturday, with "unusual seismic activity" sighted as strong aftershocks continued to jolt the remote Pacific region.

Pungent steam was reportedly rising from cracks in the ground three days after a deadly 8.0-magnitude earthquake triggered a tsunami in the region, killing at least 13 people, destroying villages and leaving thousands homeless.

A further 12 houses were reported destroyed following a 6.8 magnitude tremor and another measuring 7.0 late Friday, which sent villagers fleeing to higher ground in fear of another tsunami.

The Solomon Islands government has declared the outlying Santa Cruz Islands a disaster area as a series of aftershocks continued to hamper relief efforts.

A fragile communications system meant officials in the capital Honiara were struggling to get a clear picture of the extent of damage, but the Red Cross said food, water and shelter were a priority for quake-hit villages.

An Australian Air Force plane flew over the ravaged area on Friday and confirmed the worst damage was around the provincial capital Lata.

Authorities do not have immediate access to cargo planes capable of landing on the Lata air strip and desperately needed supplies are being shipped on a day-long journey from the Solomons capital Honiara.

"Relief operations are still going on despite the tremors and aftershocks. Water and food are the priority," Red Cross secretary general Joanne Zoleveke said.

The first vessel to arrive, a police launch carrying medical supplies, food and shelter, reached Lata on Friday evening but could not berth until Saturday morning because of the ongoing tremors.

Volcano and seismic specialists were also being called on to analyse the significance of the steam rising from the ground in parts of the Santa Cruz Islands.

"There's a lot of unusual seismic activity," a spokeswoman for the National Disaster Management Office told AFP from Honiara.

"The earth is clearly doing something there. We are asking for scientific expertise to provide us with some information about what they think might be happening. There are cracks and some steam and water coming out."

The first major aftershock on Friday "triggered a very small wave, this has caused some damage to Lata wharf", the National Disaster Management Office spokeswoman said, adding it was also hindering the unloading of emergency supplies.

A larger vessel carrying bags of rice, water and a water purifier was expected to arrive on Sunday morning.

The aftershocks have prevented villagers from returning home and they are sheltering in makeshift camps sharing limited provisions, with World Vision warning that sanitation would soon become an issue.

The Solomons are part of the "Ring of Fire", a zone of tectonic activity around the Pacific that is subject to frequent earthquakes and volcanic eruptions.

In 2007 a tsunami following an 8.0-magnitude earthquake killed at least 52 people in the Solomons and left thousands homeless.