TAINAN - Four survivors were Monday pulled alive from the rubble of an apartment complex felled by an earthquake in Taiwan, as questions intensified over why the building collapsed.

An eight-year-old girl and 28-year-old woman were the latest to be pulled from the rubble, AFP reporters at the scene said.

A man and a woman were also rescued earlier in the day as emergency workers scrabbled to find more than 100 residents still missing, over 60 hours since the building toppled. Prosecutors have launched an investigation into why the Wei-kuan building in the southern city of Tainan collapsed.

Images from the site showed metal cans and foam used inside the concrete framework of the building, raising questions over the safety of the construction.

The mayor of Tainan, William Lai, said survivors and relatives had reported building violations. The 6.4-magnitude quake left 37 confirmed dead, most of them from the apartment complex which was the only high-rise in the city to completely collapse.

Rescuers told earlier Monday how they took more than 20 hours to free one survivor, 40-year-old Lee Tsong-tian, trapped by his leg. He was eventually freed, but had to have his leg amputated.

The other survivor pulled from the rubble earlier Monday was Tsao Wei-ling, 45, who is in a stable condition - her husband and two-year-old child were pulled dead from the rubble, officials at the site said, with a search ongoing for five other members of her family trapped inside.

President-elect Tsai Ing-wen, who will take the reins in May, said the new government would prioritise building safety following the disaster.

"There are many old buildings across Taiwan... there should be an overall review of their resistance to earthquakes and other disasters," she said during a hospital visit to survivors.

Local media reported the construction company that built the complex had gone out of business and questioned the materials used.

Max Lo, former president of the Taiwan Engineering and Science Association, told AFP cans and foam could acceptably be used within decorative parts of a building to reduce its weight.

"Using them in the main structure would be against the national building code," he said.

"The first floor of the building was a shopping mall. We also need to find out if the walls designed to help bolster the building were taken out to increase the shopping space," he added.

Taiwan's President Ma Ying-jeou said there was still hope for survivors, even beyond the first 72-hour rescue window, the most likely time for people to be found alive.

"We will carry on until the last second. The golden 72 hours of rescue is the standard, but there are many exceptions," said Ma after visiting two survivors at a Tainan hospital who were in a stable condition, with fractured bones.

One of them had lost her 10-day old baby and husband. Her two other children remain missing.

The other had lost her husband, while her son and pregnant daughter-in-law are missing. "Many people are still trapped and our hearts are sinking," said Ma.

Cranes, drills, ladders, sniffer dogs and life detector equipment are being used to trace and extract the trapped, though with the building precarious, emergency workers and soldiers have also had to spend time bolstering the ruins.

Rescuers are set to start using diggers and extractors to remove giant concrete slabs once they have ensured all residents from the upper parts of the rubble have been freed. Traditionally a time of celebration and reunion, officials say many relatives would have joined their families in the Wei-kuan complex to enjoy the holidays, upping the number inside.

Now, they endure a grim wait at the site for news of the missing. Hung Yueh-yu said his brother was rescued on the first day, but his sister-in law and nephew are trapped. "I'm worried and I will keep waiting for their news - I think rescuers are working really hard. I'm hoping for the best," he said.