A series of powerful gas blasts killed at least 25 people and injured up to 267 Friday in the southern Taiwanese city of Kaohsiung, overturning cars and ripping up roads as terrified residents fled an inferno.

The explosions sparked massive fires which tore through the city's Cianjhen district, leaving a yawning trench running for hundreds of metres down the middle of a major thoroughfare and littering the streets with dead bodies.

Dramatic video footage captured by dashboard cameras inside cars showed multiple blasts and pillars of flame erupting from manholes as drivers frantically tried to avoid being engulfed.

The National Fire Agency said the blasts killed at least 25 people and injured around 267 in Taiwan's second largest city.

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 who went missing.

Premier Jiang Yi-huan inspected the affected areas and said 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 as well as a fatal air crash last week.

Chinese President Xi Jinping expressed his condolences to the victims of the gas explosions, the official Xinhua news agency said.

The blasts, believed to have been triggered by gas leaking from underground pipelines, were powerful enough to flip cars and split open paved roads.

One street had been ripped along its length, swallowing several fire engines and other vehicles, while some houses had their roofs blown off.

Workers were using cranes to lift up the vehicles and bulldozers to

clean up the scene later Friday.

Witnesses reported seeing bodies strewn across the streets of the city, which lies adjacent to a huge petrochemical complex housing dozens of petrochemical plants.

"I saw fire soaring up to possibly 20 storeys high after a blast and fire engines and cars being blown away while around 10 bodies lay on the street," witness Johnson Liu told AFP.

Local television aired footage from a dashboard camera capturing a loud explosion which tore up the road in front of a blue truck as it waited at a junction. Rocks and debris could be seen showering down on the street before the footage faded to black.

- 'Scared to death' -

Footage from a second dashboard camera uploaded online showed a car frantically making a u-turn after the initial explosion only to hurtle towards another inferno coming up from beneath the road.

"I'm scared to death," one of the occupants was recorded saying. "It's like a bombing, let's hurry."

Residents were seen carrying the injured on makeshift stretchers as ambulances rushed to the scene and firefighters in yellow overalls began removing bodies from the area.

"The explosions were like thunder and the road in front of my shop ripped open. It felt like an earthquake," Taiwan's Central News Agency quoted a witness as saying.

"The local fire department received calls of gas leaks late Thursday and then there were a series of blasts around midnight affecting an area of two to three square kilometres," the fire agency said in a statement.

A Kaohsiung city government official said the blazes had been extinguished or burned themselves out. City authorities said they had sealed off six kilometres (about four miles) of road.

Residents described how the neighbourhood smelt strongly of gas before the disaster.

One surnamed Peng said: "There was a heavy odour of gas and... then I heard explosions and saw fire spurting from a store."

"My house shook as if there were an earthquake and the power went out," she was quoted as saying by the Central News Agency.

The local government was evacuating more than 1,100 residents from the affected areas to schools and shelters as it tried to locate the source of the leaks and warned people to stay away.

The military dispatched around 1,400 soldiers to the scene to help with the disaster effort.

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.

Friday's 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.