Shanghai -

Shanghai was forced to deny that China's future tallest building, now under construction, was on fire on Thursday after what looked like smog surrounded the towering structure.

The incident came after a factory fire in the neighbouring province of Zhejiang went unnoticed for three hours earlier in January because of thick haze, according to state media.

Alarmed microblog users posted photos of the Shanghai Tower, which will stand 632 metres tall after it is topped out later in 2013, with smoke appearing to billow from the building.

“It really looks like it's on fire,” said a microblogger using the handle Jolly Baobei.

China's commercial hub has been less affected than much of the country by the heavy pollution of recent weeks, although air quality hit “very unhealthy” levels on Wednesday, according to the US consulate in the city.

The Shanghai government sought to calm the panic by denying a blaze, but its explanation of “fog” was quickly ridiculed.

“We have confirmed with the police and fire department, the Shanghai Tower is not on fire. Internet friends are mainly seeing today's low cloud layer caused by the surrounding fog,” the city authorities said.

But Internet users said the culprit was air pollution. One, referring to the tiny air particles that can deeply penetrate the lungs, said: “It's PM 2.5, which is fiercer than fire.” - Sapa-AFP