BEIJING, Jan. 7 (Xinhua) -- A strong cold front late on Sunday is expected to disperse smog that has lasted for several days in north China, the country's meteorological authorities said Saturday.

The smog, which has covered the region since last weekend, will be dispersed from north to south, China Meteorological Administration (CMA) said in a statement posted on its official website.

The CMA extended its orange alert on Saturday as smog and heavy fog continued to blanket northern and eastern China, with some regions seeing heavy pollution.

China has a four-tier warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.

More than 20 cities in north China issued red alerts for the latest bout of polluted air.

Hundreds of flights were disrupted and many freeways were closed due to low visibility in affected areas.