Rescuers raced against time Sunday to free more than 100 people buried beneath the rubble of apartment blocks felled by an earthquake in southern Taiwan that left 19 dead, as an investigation was launched into the building collapse.

The death toll is rising as emergency workers dig to find survivors of the 6.4-magnitude quake that toppled the 16-storey apartment complex containing almost 100 homes in the city of Tainan on Saturday.

Officials said an investigation had been launched as questions were raised over the safety of the residential blocks.

Tainan mayor William Lai said survivors and relatives had reported there were legal "violations" at the building, without giving any further detail.

"I've contacted judicial units and prosecutors have formally launched an investigation," said Lai.

"We've also commissioned three independent bodies to preserve evidence during the rescue so we can assist the residents if they want to file lawsuits in the future. We will hold the builder responsible if they have broken the law."

Local media reported the construction company that built the complex had gone out of business and also raised questions over the quality of the materials used.

Rescuers said 124 residents were still missing, with 103 of them trapped "very deep" in the rubble, according to Lai.

"There's no way to get to them direct, it's very difficult," Lai said, adding that emergency workers were having to bolster the ruins to ensure they were secure before digging.

Census records show around 260 people living in the blocks but Lai said it was now thought that more than 300 had been inside.

Officials have said that some students renting rooms would not have been registered as living in the building, and additional family members may have returned there to celebrate next week's Lunar New Year holidays.

- Agonising wait -

More than 250 have already been rescued with emergency workers using cranes, ladders and sniffer dogs to trace and extract survivors, with three more brought out alive Sunday morning.

Among the 19 people killed by the quake, 17 died in the apartment complex collapse, including a 10-day-old baby girl and two other children.

Tearful relatives huddled by the ruins, hoping for news.

One woman, Chen Yu-mei, told how her three-month-old son was trapped inside with her sister's family of seven.

"He's a small child, I'm worried sick," she told AFP, her voice cracking and eyes red from crying.

"I've been waiting since 4:00 am yesterday and I haven't got any news of them."

Meanwhile, survivors spoke of their terror and relief.

One man told AFP how he was rescued after tapping on a wardrobe that was trapping him.

"I knocked on the closet to get the attention of rescuers who broke the window to get me," said resident Su Yi-ming, 48, who lived with his family on the sixth floor of the Wei-kuan complex.

Su escaped uninjured, with his wife and their two children sustaining minor injuries.

The quake struck at a shallow depth of 10 kilometres (six miles) at around 4:00 am Saturday morning (2000 GMT Friday), 39 kilometres northeast of Kaohsiung, Taiwan's second-largest city.

Taiwan lies near the junction of two tectonic plates and is regularly hit by earthquakes.

The island's worst tremor in recent decades was a 7.6 magnitude quake in September 1999 that killed around 2,400 people.