PALU: Indonesian volunteers began burying bodies in a mass grave with space for more than a thousand people on Monday, victims of a quake-tsunami that devastated swathes of Sulawesi and left authorities struggling to deal with the sheer scale of the disaster.

Indonesia is no stranger to natural calamities and Jakarta had been keen to show it could deal with a catastrophe that has killed at least 844 people, according to the latest official count, and displaced some 48,000 more.

But four days on some remote areas are only now being contacted, medicines are running out and rescuers are struggling with a shortage of heavy equipment as they try to reach desperate victims calling out from the ruins of collapsed buildings.

In response, President Joko Widodo opened the door to the dozens of international aid agencies and NGOs who are lined up to provide life-saving assistance.

"Last night, President @jokowi authorized us to accept international help for urgent disaster-response & relief," senior government official Tom Lembong wrote on Twitter on Monday.

Officials fear the toll will rise steeply in the coming days and are preparing for the worst, declaring a 14-day state of emergency.

At Poboya -- in the hills above the devastated seaside city of Palu -- volunteers began to fill a vast grave with the dead, with instructions to prepare for 1,300 victims to be laid to rest.

Authorities are desperate to stave off any disease outbreak caused by decomposing bodies, some now are riddled with maggots.

Three trucks arrived stacked with corpses wrapped in orange, yellow and black bags, an AFP reporter on the scene saw. One-by-one they were dragged into the grave as excavators poured soil on top.