A powerful earthquake struck Taiwan today killing at least seven people, most in a 17-storey apartment building that collapsed, with about 35 people still feared trapped there hours into a rescue operation.

As a search for survivors in the rubble went on, questions were being raised about the construction of the Wei-guan Golden Dragon Building, with its floors that pancaked down on each other when the 6.4 magnitude tremor hit at around 4am, at the start of a Lunar New Year holiday.

Five of the dead, including a 10-day-old girl, were from the apartment building in the southern city of Tainan.

Rescuers mounted hydraulic ladders and a crane to scour the ruins, plucking more than 220 survivors to safety, with dozens taken to hospital, a fire brigade official said.

Buildings in nine other locations in the city of 2 million people had collapsed and five were left tilting at alarming angles, a government emergency centre said.

But a fire department official said rescue efforts were focused on the apartment block, where a child's clothes fluttered from a first-floor laundry line and the smell of leaking gas hung in the air.

"I was watching TV and after a sudden burst of shaking, I heard a boom. I opened my metal door and saw the building opposite fall down," said a 71-year-old neighbour who gave his name as Chang.

A plumber, he said he fetched some tools and a ladder and prised some window bars open to rescue a woman crying for help.

"She asked me to go back and rescue her husband, child, but I was afraid of a gas explosion so I didn't go in. At the time there were more people calling for help, but my ladder wasn't long enough so there was no way to save them."

The quake was centred 43km south-east of Tainan, at a depth of 23km, the US Geological Survey said.

An elderly woman, wrapped in blankets, was strapped to a board and slowly slid down a ramp to the ground as the cries of those still trapped rang out. Rescuers used dogs and acoustic equipment to pick up signs of life in the rubble.

Authorities said there were 96 units and 256 people living in the collapsed apartment building, but they did not know how many were actually there when the quake struck.

The fire department said 115 people had been taken to hospital from around Tainan.

Several buildings damaged

City officials said it was too early to determine if poor construction was a factor in the collapse.

Liu Shih-chung, city government deputy secretary general, said television footage of the ruins of the commercial-residential building suggested the possibility of structural problems related to poor-quality reinforced steel and cement.

The construction and engineering companies that built the complex are no longer operating, records showed.

Two neighbours said they had felt nervous about the construction when the building was going up in the 1990s.

"I looked at it and thought, only people from out of town would buy there. We local people would never dare," said one of the neighbours, Yang Shu-mei.

A major earthquake in central Taiwan in 1999 killed about 2,400 people and caused damage across the island, which lies in the seismically active "Pacific Ring of Fire".

President Ma Ying-jeou visited an emergency centre and hospital in Tainan while president-elect Tsai Ing-wen cancelled appointments to help coordinate rescue efforts.

China's Taiwan Affairs Office, which is in charge of Beijing's relations with the self-ruled island, said China was willing to provide help if needed, Chinese state news agency Xinhua said. Beijing regards Taiwan as a wayward province.

The quake initially cut power to 168,000 households in Tainan though utility Taiwan Power Co later said electricity had been restored to all but about 900 households.

Taiwan Semiconductor Manufacturing Co (TSMC), the world's largest contract chipmaker, said some of its wafers made in Tainan had been damaged and some customers might be affected, though first-quarter shipments remained on track.

TSMC will step up production to make up for any delayed shipments, spokesperson Elizabeth Sun said. It is a big supplier to global smartphone firms, including Apple Inc.

Other major Apple suppliers in Taiwan reported no impact on operations.

- Reuters