The official death toll is 215, but officials fear that more people than that have died, says the BBC's Charles Haviland in Islamabad.
Many people buried their close relatives soon after their deaths, making any accurate count difficult.
A health officer in Ziarat, the worst-affected area, has also stressed that some remote villages have not yet been has surveyed.
Aftershocks
Some survivors say that many villages away from main roads have only slowly and belatedly been receiving food, blankets and tents from the relief teams sent by the government.
The quake crushed hundreds of vulnerable houses
Military helicopters are being used to reach mountainous and remote locations, where some villages have been cut off by landslides.
"The earthquake destroyed our houses, but now the government's slow response is killing us," Moosa Kaleem, a survivor in Ziarat, told the Associated Press news agency.
"We cannot spend another night in this chilling weather, especially the kids."
Several hundred aftershocks have been felt since the main earthquake.
"I know these are aftershocks and not new earthquakes, and I also know these tremors may continue for a while but it is hard to convince children that they will be safe," Amjad Aziz, a teacher in Ziarat who has been sleeping in his car since the quake, told AP.
The aftershocks have also been felt in the city of Quetta.
There, hospital patients - including people injured in the quake - are lying on the ground or on beds in the open air, as staff do not consider it safe to stay indoors.
The shocks are still causing widespread alarm, and some people have left the city for rural areas.
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Villagers remain in tents after fears of an aftershock
Activists with hard-line Islamic organisations were first to reach some of the most remote parts with relief supplies.
Among them was Jamaat-ud-Dawa, designated a terrorist group by the US government for its links to Kashmiri Muslim separatists.
The same group also helped survivors of a huge quake that devastated northern Pakistan in October 2005, killing up to 80,000 people.
Balochistan is home to a long-running separatist movement, but has so far been spared the level of violence seen in the north-western tribal areas bordering Afghanistan.
In 1935, Quetta suffered almost complete destruction in an earthquake which claimed the lives of about 30,000 people.
Pakistan sits atop an area of seismic collision between the Eurasian and Indian tectonic plates, the same force that created the Himalayan mountains.
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