"A video posted by the Taiwanese broadcaster TVBS Taiwan showed the moment of the explosion, with cladding and other debris flying off the building and dust billowing from inside. Photos from the scene showed scattered fragments strewn on the streets.

"I was working in the city government building around 11:30am and because it's right next to Shin Kong Mitsukoshi department store, I felt a vibration," Taichung Mayor Lu Shiow-yen told reporters.

It is not yet known what caused the blast.

"The cause is subject to follow-up investigation and we call on the public to avoid going to the vicinity of the disaster site," the National Fire Agency said.

Lu said the explosion had affected a wide area and at least one passer-by was among the victims.

"There are many office buildings and homes in this area and we ask everyone to cooperate with the restrictions for your safety," he said.

"The explosion is very serious and the rescue is ongoing."

It is not yet known what caused the blast.

"The cause is subject to follow-up investigation and we call on the public to avoid going to the vicinity of the disaster site," the National Fire Agency said.

Lu said the explosion had affected a wide area and at least one passer-by was among the victims."