Taiwan's president promised on Saturday to launch a swift investigation into the cause of deadly gas explosions that rocked the island's densely populated second city, as residents accused local authorities of failing to avert the tragedy.

The blasts sparked massive fires which tore through Kaohsiung's Cianjhen district late Thursday, killing at least 28 and leaving a yawning trench running for hundreds of metres down a series of major thoroughfares in the worst gas explosion ever in Taiwan.

Cars were thrown onto the roofs of buildings several stories high and had to be winched down to earth, and nearly 300 people suffered injuries, including serious burns, during the explosions and subsequent inferno.

President Ma Ying-jeou, who visited a Kaohsiung hospital where the injured were recuperating on Saturday, vowed a full investigation into the cause of the incident, and a review of the network of subterranean pipes.

"Everyone is concerned about the cause of the incident and the cabinet has set up a task force to investigate and hope to find the cause in the shortest possible time," he told reporters.

"Even though this incident happened in Kaohsiung, every Taiwanese person's heart is hurt... (I) hope heaven blesses Kaohsiung and blesses Taiwan."

As rescuers continued the grim search for human remains buried under the rubble, many residents had accused the southern city's authorities of acting too slowly to stop the gas leaks, which locals said were first reported three hours before the explosions struck shortly before midnight.

"I think the authorities didn't handle this well, if they had turned off the pipelines right away there would not have been any explosion that followed," local resident Lin Chung-hua told AFP.

"The tragedy could have been prevented if the source of the leak was located at the first possible time and the pipelines shut down," added a 60-year-old restaurant owner who declined to give his name.

Ma said that "more comprehensive design and efficient management" would be the focus of a review of the pipe network.

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Kaohsiung lies adjacent to a huge petrochemical complex housing dozens of plants and many pipelines run under the densely packed city.

Lin, whose shop lies on one of the streets torn apart by the blast, said he wanted the pipelines relocated.

"The pipelines were installed many years ago when our neighbourhood was not densely populated but now there are so many people living here. Of course they have to be removed because they are unsafe," he said.

Kaohsiung's city government has defended its handling of the crisis, saying it was "racing against time" and had carried out the correct measures once the leaks were reported.

- Firefighters still missing -

Officials have said some evidence indicated that a propene leak from the pipe system of LCY Chemical Corp. might have caused the multiple gas explosions. The company has pledged to cooperate with the investigation into the incident.

The United Daily News quoted an unnamed firefighter who said the pipelines should have been shut down sooner.

"The blasts wouldn't be so devastating if the (pipelines) valves were shut earlier," he was quoted as saying.

Four firefighters who rushed to the scene after residents smelled gas were among those killed in the blasts while rescuers were searching for two others still unaccounted for.

Kaohsiung mayor Chen Chu said the tragedy had "shocked residents tremendously" as she called on the central government to review the city's network of pipelines.

"I instructed relevant units to thoroughly inspect the pipelines and call for the central government to review how to properly locate them so residents do not live under invisible threats and to prevent another tragedy," she said in a statement.

Religious rituals were held at the disaster site for grieving relatives to appease the spirits of their loved ones, as they held incense sticks while monks chanted prayers.

Rescuers and soldiers found two bodies buried under rocks and debris earlier Saturday. One was identified as a 75-year-old man while the other's identity was yet to be confirmed.

It is not the first time Kaohsiung has experienced a fatal gas blast. In 1997, an explosion killed five people and injured around 20 when a team from the state-run Chinese Petroleum Corp. tried to unearth a section of gas pipeline in a road construction project.

The latest explosions were the second disaster to strike Taiwan in just over a week, after a TransAsia Airways plane crashed with the loss of 48 people last Wednesday.

Flags would be flown at half-mast at government offices and schools across the island from August 5 for three days to mourn the victims of the blasts and the air crash.