Hazy weather hung over Beijing on Wednesday with conditions in other parts of north China possibly worsening, forecasters said, although cold air sweeping in from the northwest is expected to help dissipate the smog.

Experts attributed the haze in Beijing, which has lasted for several days, to poor atmospheric diffusion because of unseasonably weak cold air currents from the north.

Visibility in most parts of Beijing dropped to less than 500 metres (546 yards), the China Meteorological Administration said.

"I didn't expect the smog to be so bad these days. The masks I've brought are almost used up," said a 24-year-old Beijing tourist surnamed Gao.

In some places, heavy winter fog was adding to the murky conditions and visibility was less than 50 metres in some parts of Hebei province and Tianjin city.

Moderate to heavy air pollution was expected in the Bejing-Tianjin-Hebei area and surrounding cities until Thursday, with short bouts of severe conditions, the environment ministry said after a meeting of its agencies.

While the north of the region will improve in the first half of this month, pollution may persist in southern areas, it said.

"The overall environment is getting better these years but in recent days the smog has come back. We're not sure what's exactly causing it," said an 82-year-old Beijing resident surnamed Liu.

Unusually mild winter temperatures, with some places recording 10 degrees Celsius above normal, were likely to persist until Thursday, forecasters said, with records likely to be broken in big cities such as Jinan, Zhengzhou and Shijiazhuang.

But from Thursday night, cold air is expected to push temperatures lower, possibly to new lows. (Reporting by Liz Lee, Ethan Wang and Xiaoyu Yin; editing by Robert Birsel)