KAOHSIUNG, Taiwan -- Hundreds of people who fled from gas pipeline explosions in Taiwan's second-largest city returned to their homes Friday after authorities said there was no more risk of blasts like the series that ripped apart streets overnight, killing 26 people and injuring 267.

With clean-up work underway in the 2-square kilometre (1-square mile) area, investigators were turning to the task of determining the cause of the blasts, the industrial city's worst such disaster in 16 years.

Most of the four ruptured street sections in the densely populated district of Kaohsiung were declared safe from further explosions by afternoon, a city spokesman said. A fire in a 10-meter (yard) -long section that burned through the night had also been put out.

Five explosions ripped through four streets starting around midnight Thursday, catapulting cars into the air and blasting cement rubble at passers-by, many of whom were out late at a nearby night market.

That came about three hours after a gas leak had been reported on Kaixuan Road, but emergency services had been unable to locate the source.

Four firefighters were among the victims and two were missing, while at least six fire trucks were flung into the rubble. The blasts sent flames shooting into the sky and left broad, meter-deep (yard-deep) trenches down the middle of roads.

Many of the injured were still receiving medical treatment. The disaster was Taiwan's second in as many weeks following the crash of a TransAsia Airways prop jet on the island of Penghu on July 23 that killed 48 people and injured 10.

"Last night around midnight, the house started shaking and I thought it was a huge earthquake, but when I opened the door, I saw white smoke all over and smelled gas," said Chen Qing-tao, 38, who lives a short distance from the devastation.

The explosions were believed caused by leaking propene, a petrochemical material not intended for public use, said Chang Jia-juch, director of the Central Disaster Emergency Operation Center.

Propene is mainly used for making the plastic polypropylene used in a wide variety of packaging, caps and films. It can be detected by its mildly unpleasant smell.

The city's Environmental Protection Bureau director told Taiwan's Central News Agency that propene was leaking from the gas pipes that exploded. It originated from a warehouse used by China General Terminal & Distribution Corp., which stores and transports petrochemical raw materials, Director Chen Chin-der told the news agency.

Industrial-use pipelines run through Kaohsiung's residential neighbourhoods because industry preceded the construction of houses, said city spokesman Ting Yun-kung. The port city contains much of Taiwan's heavy industry, especially petrochemicals.

Video from broadcasters showed residents searching for victims overnight in shattered storefronts and rescuers placing injured people on stretchers. Numerous fires sent smoke pouring into the night sky above the Chian-Chen district, where factories operate near low-rise residential buildings.

The government's disaster response centre spent much of Friday trying to prevent secondary explosions. With the risk easing after midday, all but 300 evacuees had left emergency shelters and just one of an initial nine remained open, Ting said.

The Central News Agency's English-language service said 12,000 people were sent to emergency shelters, but its Chinese service put the figure at 1,200, which is close to the city's earliest estimates.

Area resident Chang Bi-chu, 63, described seeing dead bodies along the roadside. "I felt really bad. After all, there just was the air crash in Penghu last week."

Chang said the front door of her home was warped by the explosion and power was cut, leaving the house without lights or fans in the steamy weather.

"We don't have money to stay in a hotel and they're all booked anyway," she said.

Power supplies to 12,000 people in the area were severed, and 23,600 lost gas service. Some power had been restored by late Friday.

Backhoes pulled upended fire trucks and other vehicles from the rubble while paramedics with rescue dogs combed the neighbourhood for survivors.

Rescuers expected to find few, if any, people in the rubble because no buildings collapsed, said Hsu Lee-hao, a national emergency operations centre official.

Large trenches edged with pavement slabs torn apart by the blasts dominated the area, which was cordoned off most of Friday. Burned walls and toppled shop signs lined Sanduo Road, near an elementary school. Television images showed one car thrust onto a building roof by a particularly powerful blast.

Taiwanese Premier Jiang Yi-huah announced that all flags would fly at half-staff for three days from Aug. 5 in honour of the victims of both the Penghu air crash and Kaohsiung explosion. President Ma Ying-jeou paused at a scheduled event Friday morning to call for a minute of silence.

Much of the drama was captured on closed-circuit television, dashboard cameras and cellphones.

A video showed an explosion rippling through the floor of a motorcycle parking area, hurling concrete and other debris through the air. Cellphone video captured the sound of an explosion as flames leapt at least 9 metres (30 feet) into the air.

One witness said he tried to help before paramedics arrived.

"I was on my scooter just across the street, suddenly there was the explosion, a white car was blown toward me, and I saw the driver trapped in the car," said Wong Zhen-yao, 49, owner of a car repair shop in the disaster area.

"There was still fire nearby. I tried to pull the guy out but couldn't," he said. "Only after the smoke was gone did I realize there was such a big hole in the middle of the road."