* Emergency declared across Highlands provinces - PM
* PNG government, Red Cross, Australia pledge quake aid
* Rescuers and supplies yet to reach hard-hit areas - official
SYDNEY, March 2 (Reuters) - Papua New Guinea has declared a state of emergency across its remote and rugged highlands, the country's Prime Minister Peter O'Neill said on Friday, days after a deadly quake struck the region and killed at least 31 people.
Rescuers have struggled to reach villages buried by landslides near the epicentre, and even hard-hit larger towns, since the 7.5-magnitude quake wrecked roads and runways on Monday and cloudy weather made helicopter flights difficult since.
"This is an unprecedented disaster," O'Neill said in a statement late on Wednesday, while promising 450 million kina ($135 million) in government aid and help from the military.
"A state of emergency has been declared to expedite the restoration of essential public services including healthcare services, schools, road access, airports, power and communications facilities," he said.
At least 13 people died when landslides covered remote hamlets close to where the quake struck, some 560 km (350 miles) northwest of the capital, Port Moresby, an official told Reuters on Thursday.
While the region has no major urban centres, around 670,000 people live within 100 km of the epicentre according to the Red Cross.
Most of the other confirmed fatalities were in or around the provincial capital of Mendi and the town of Tari 40 km (25 miles) from the epicentre, where airports are closed and relief workers are yet to arrive.
"Our people live in scattered hamlets and people are dying slowly - the only means of rescue is through helicopters and they are hardly coming," Hela province's administrator, William Bando, told Reuters from his office in a shipping container in Tari.
"A lot of people are asking for tents, water and medical supplies. It's a beautiful day here today, we hope a helicopter comes."
The International Federation of Red Cross and Red Crescent Societies said it released $221,000 in funds to help relief efforts and would send first aid, water, mosquito nets and shelters, to the region.
Australia has also promised aid and sent a C-130 military plane to help with aerial surveys.
Miners and oil and gas companies were also assessing damage to their infrastructure, including a 700-km (435-mile) gas pipeline that connects to a coastal liquefaction plant, which ExxonMobil Corp on Thursday said was not damaged.
Earthquakes are common in Papua New Guinea, which sits on the Pacific Ocean's "Ring of Fire," a hotspot for seismic activity due to friction between tectonic plates. ($1 = 3.3333 kinas)
(Reporting by Tom Westbrook, editing by G Crosse)