About 1,000 people are still missing in three areas of Indonesia following last week’s earthquake and tsunami, Sky News has learned.
A spokesman said that most victims in Petobo are in flattened homes buried under at least three metres of mud that has now solidified.
Sutopo Purwo Nugroho said: “The liquefaction damage is unprecedented in Indonesia, despite high seismic activity.”
It comes as the number of dead rose to 1,424, while more than 70,000 have been left homeless.
Thousands more are believed to be missing from other areas, with people trapped under rubble or mud due to the process of liquefaction, which sees the earth loosen due to the earthquake then turn into what looks like a heavy liquid.
Authorities have set a tentative deadline of Friday to find anyone still alive under the rubble. At that point, a week after the disaster, the chances of finding survivors is almost zero.
The 7.5-magnitude earthquake hit Sulawesi island on Friday and was followed by a tsunami as high as 6m (20ft) which destroyed homes and left hundreds of thousands desperate for food and water.
Six days after disaster struck, survivors are begging for handouts and looting shops as aid has been slow to get to many areas.
The Disasters Emergency Committee (DEC), which brings together 14 UK aid charities, has launched an appeal to raise money for survivors.
DEC chief executive Saleh Saeed said the charities and local partners were working with Indonesian authorities “to get aid to those who urgently need it, as well as helping survivors to cope with the trauma of the last few days”.
He added: “There is an urgent need for clean drinking water, food, medical care and shelter.”
“Please give generously and let’s save the survivors.”
The DEC has set up a number – 70000 – where people can easily donate £5 by texting SKY.
The UK government is also sending thousands of shelter kits, solar lanterns and water purifiers to the disaster zone, with a loaded plane leaving the UK on Thursday.
Six experts have been sent to Sulawesi to help coordinate the aid effort and the government offered to send HMS Argyll from Singapore to help.
Indonesian authorities said the ship was not yet needed.
Ruined bridges, damaged roads and landslides have slowed down the rescue and aid effort, says Sky News’ Mark Stone, who is in the disaster zone.
He said that so far he had not seen “any coordinated government response” – despite the UN saying 200,000 people urgently need help.
A lack of heavy machinery to move the rubble has also caused problems, while there are safety concerns for those trying to help after about 1,200 inmates escaped from two prisons.
Some services in Palu, a small city of 370,000 people, have started to return to normal – with banks reopening and a major mobile phone network resuming service. Fuel shipments have also started to arrive.
:: Donations can be made at www.dec.org.uk, on the 24-hour hotline on 0370 60 60 900; or by texting the number mentioned above. People can also give money over the counter at any high street bank or post office.