Twitter has been blocked in Turkey amid mounting anger over the response to the earthquake as President Yayyip Erdogan insisted it was “not possible” to prepare for a disaster of this magnitude.

The death toll rose to more than 11,500 across Turkey and Syria on Thursday morning (AEDT), making it the world’s deadliest seismic event in a decade.

In 2011, a huge magnitude 9 earthquake triggered a tsunami off Japan, killing 20,000 people.

Mr Erdogan visited southern Turkey to see first-hand the destruction as criticism grew over what local people said was a slow government response to the rescue and relief effort.

The tally in Turkey and Syria is expected to rise as collapsed buildings become tombs for people trapped for days under the rubble in freezing temperatures.

Foreign Minister Penny Wong confirmed four Australians remained unaccounted for in Turkey.

Social media posts had been filled with people complaining about the lack of help in their provinces as people were forced to use whatever tools they could to dig.

As anger spread, social media monitoring group said Twitter was being blocked across all major mobile phone providers.

“The filtering measure is likely to impact community rescue efforts underway after the series of deadly earthquakes on Monday,” netblocks.com warned.

“Turkey has an extensive history of social media restrictions during national emergencies and safety incidents.”

France24 reported that Turkish police had detained 18 people over “provocative” social media posts that criticised the government’s response.

Families in southern Turkey and in Syria spent a second night in the freezing cold as overwhelmed rescuers tried to pull people from the rubble.

Many in the Turkish disaster zone had slept in their cars or in the streets under blankets, fearful of going back into buildings shaken by the 7.8 magnitude tremor – Turkey’s deadliest since 1999 – and by a second powerful quake hours later.

The death toll rose above 8500 in Turkey.

In Syria, already devastated by 11 years of war, the confirmed toll climbed to more than 2500 overnight, according to the Syrian government and a rescue service operating in the rebel-held northwest.

Mr Erdogan, who has declared a state of emergency in 10 provinces and sent in troops to help, arrived in the city of Kahramanmaras to view the damage and see the rescue and relief effort.

The disaster poses a new challenge to him in the election he faces in May that was already set to be the toughest of his two decades in power.

Any perception that the government is failing to address the disaster properly could hurt Mr Erdogan’s prospects in the vote, but analysts say that on the other hand, he could rally national support around the crisis response and strengthen his position.

Reuters journalists in Kahramanmaras saw about 50 bodies draped in blankets on the floor of a sports hall. Family members searched for relatives among the dead.

Kneeling on the auditorium floor, a woman wailed with grief and embraced a body wrapped in a blanket

In Hatay province, where dozens more bodies lay outside in rows between Red Crescent tents, people opened body bags hoping to identify loved ones.

The quake toppled thousands of buildings including hospitals, schools and apartment blocks, injured tens of thousands, and left countless people homeless in Turkey and northern Syria.

Turkish authorities say some 13.5 million people were affected in an area spanning roughly 450 kilometres from Adana in the west to Diyarbakir in the east.

In Syria, it killed people as far south as Hama.

Turkey’s disaster management agency said the number of injured was above 38,000.

Aid officials voiced particular concern about the situation in Syria, where humanitarian needs were already greater than at any point since the eruption of a conflict that has partitioned the nation and is complicating relief efforts.

Residents in Syrian government-held territory contacted by phone have described the authorities’ response as slow, with some areas receiving more help than others.

In the town of Jandaris in northern Syria, rescue workers and residents said dozens of buildings had collapsed.

Standing around the wreckage of what had been a 32-apartment building, relatives of people who had lived there said they had seen no one removed alive.

A lack of heavy equipment to remove large concrete slabs was impeding rescue efforts.

Rescue workers have struggled to reach some of the worst-hit areas, held back by destroyed roads, poor weather and a lack of resources and heavy equipment. Some areas are without fuel and electricity.

A rescue service operating in insurgent-held north-west Syria said the number of dead had climbed to more than 1280 with more than 2600 injured.

-Reuters