"We think potentially the impacted individuals would go up to almost into the hundreds of thousands," Indrika Ratwatte, the UN humanitarian coordinator in Afghanistan, told reporters in Geneva, speaking from Kabul.

Thousands are known to have been injured in the 6.0-magnitude earthquake, which hit remote areas in mountainous provinces near the border with Pakistan around midnight Sunday, followed by at least five aftershocks.

"The numbers are definitely going to increase," Ratwatte said.

"There is no question that the casualty rate is going to be rather exponential".

The majority of Afghans live in low-rise, mud-brick homes vulnerable to collapse.

Ratwatte said homes in the affected region were largely "mud- and wooden-roof structures, so when the walls collapse the roof is what basically for the individuals kills them or suffocates them".

Given the timing, "everybody was sleeping, so I think (the casualty figure) is going to be much higher".

He added that the quake had set off "lots of landslides, rockfalls", with limited access posing "a huge challenge".

"The biggest challenge is to reach these remote areas with the road access extremely damaged," he said, stressing the need for helicopters to reach those affected, evacuate the injured and deploy search and rescue and medical teams.

After decades of conflict, Afghanistan is one of the world's poorest countries, facing a protracted humanitarian crisis deepened by severe drought and the influx of millions of Afghans forced back to the country by neighbours Pakistan and Iran in recent years.

UN refugee agency spokesman Babar Baloch said that of the more than 478,000 Afghans who have returned from Pakistan since April, "around 337,000... passed through the Torkham border crossing, close to the epicentre of the earthquake".

"Some 24 percent of returnees from Pakistan came to Nangarhar Province, one of the hardest hit by the earthquake," he told reporters.

"These people, already with very little resources, are now returning to a disaster zone."

Ratwatte meanwhile warned that this year's appeal for $2.4 billion to provide desperately needed aid to Afghans facing multiple crises was so far only 28-percent funded.

"We need a major, urgent step up by the international community to respond to this crisis," he said.