The United Nations emergency relief coordinator has warned that the death toll from the powerful quake in Tukey and Syria will possibly double the current number.

"I think it is difficult to estimate precisely as we need to get under the rubble, but I'm sure it will double or more,” Martin Griffiths said.

"It's deeply shocking... the idea that these mountains of rubble still hold people, some of them still alive,” he said, adding that "we haven't really begun to count the number of dead."

Griffiths arrived on Saturday in Turkey's southern city of Kahramanmaras, the epicentre of the first 7.8-magnitude tremor that upturned millions of lives on Monday.

With every passing moment, hopes are fading for those trapped under wreckage in Turkey, Griffiths said, adding that rescue efforts were now in their final stages in Syria.

"They say 72 hours is the golden period (for rescues)," Griffiths said.

"But just now they have pulled out someone alive, an hour or two ago. It must be incredibly difficult to decide when to stop this rescue phase and move into the next phase, which is also going to have its problems," he added.

According to officials and medics, so far 24,617 people were killed in Turkey and 3,574 in Syria, summing up to a confirmed total death toll of 28,191.

Tens of thousands of rescue workers are excavating through flattened neighborhoods and debris in freezing temperatures that have deepened the misery of millions now in desperate need of aid.

There have been more than 32,000 disaster management personnel from Turkish organizations deployed at the affected areas and are working on search-and-rescue efforts, Turkey's disaster agency said.

Apart from the national rescuers, there are also 8,294 international rescuers present.

"Soon, the search and rescue people will make way for the humanitarian agencies, whose job is to look after the extraordinary numbers of those affected for the next months," Griffiths said in a video posted to Twitter.

Around 26 million people have been affected by the earthquake, the World Health Organization (WHO) said as it launched a flash appeal on Saturday for $42.8 million to cope with immediate and overwhelming health needs.

The United Nations has warned that at least 870,000 people urgently need hot meals across Turkey and Syria. Up to 5.3 million people may have been made homeless in Syria alone.

In Syria, conditions are even worse, as the country has been hit by years of war, and adding to the challenges, the harsh American sanctions create difficulties in getting aid into the Arab country.

Griffiths said it is "much more difficult for [Syria] to cope with this tragedy because of those desperate lost years".

"The awful truth about Syria from a humanitarian perspective is that the needs of the people of Syria grow each year and despite generous funding, we fail to meet those needs. As a result, the people who live in absolute poverty in Syria grows - now 90 percent of the population," he added.