A magnitude 6.8 aftershock has killed at least 18 people in Papua New Guinea, bringing the total number of deaths to more than 100 since a powerful earthquake struck the southwestern Pacific island state in late February.

The aftershock was one of the six strong quakes that hit the country since midnight on Wednesday, Australian Broadcasting Corporation reported.

According to The Sydney Morning Herald, the 18 fatalities were from the province of Hela north of the capital Port Moresby.

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DPA news agency also reported seven more deaths in two separate districts.

The country is still dealing with the aftermath of the magnitude 7.5 earthquake that hit on February 26, which brought widespread destruction to infrastructure and left thousands homeless.

State of emergency

Prime Minister Peter O'Neill had declared a state of emergency on March 1 in the disaster-stricken provinces.

So far, the United Nations has released $1.4m in aid to the victims, covering medical services and emergency shelters. Australia has also committed aid close to a million dollars.

The Papua New Guinea government has allocated $138m for relief supplies to victims and for the restoration of basic services, according to the country's The National newspaper.

The newspaper reported that, in some areas, help has yet to reach the survivors nine days after the disaster.

"We are yet to fly one bale of rice to one particular location; we're yet to take medication, water. No shelter, no nothing," Eric Aliawi, a helicopter pilot involved in the disaster relief, told The National.

Landslides and damaged roads and bridges have made it impossible to reach some of the areas by land.

On Tuesday, The National reported that in Southern Highlands province, 11 people were buried by landslides caused by the earthquake.

Basic supplies and emergency kits were also reportedly flown into the disaster areas with the aid of the Australian government.

Papua New Guinea, which has a population of seven million, is located on the eastern half of the island of New Guinea, east of Indonesia.

Like many Pacific islands, Papua New Guinea is located along the so-called "Ring of Fire", which stretches in a horseshoe shape around the Pacific Ocean, and where most quakes and volcanic activities are recorded.