BEIJING (AFP) - A factory fire in eastern China went unnoticed for three hours because of the thick smog that blanketed large stretches of the country this week, state media said on Tuesday.

After firefighters in Zhejiang province were finally alerted to the blaze in the early hours of Monday they took 10 hours to extinguish the flames, which destroyed a large amount of furniture, the Xinhua news agency said.

"Because of the thick fog pervading the air at the time, the initial smoke and flames produced by the fire took an unexpected almost three hours to be discovered by nearby residents," Xinhua said.

The unusually dense air pollution across much of China this week reached levels dangerous to human health and prompted public criticism of the nation's breakneck development.

At the height of the smog authorities in the capital said readings for PM2.5 - particles small enough to penetrate the lungs - hit 993 micrograms per cubic metre, almost 40 times the World Health Organisation's safe limit.

The reduced visibility contributed to a 20-car pileup elsewhere in Zhejiang on Tuesday that left two people dead and eight injured, Xinhua said in a separate report, and forced flights in Beijing to be cancelled.