A heavy fog that has affected most parts of Tianjin since Wednesday forced the closure of many expressways that link the northern China municipality with neighboring provinces, city authorities said on Thursday.

Wednesday's mist worsened into a heavy fog overnight and reduced visibility to 20 meters, the municipal meteorological observatory said.

For safety reasons, the local road transport administration closed at least three trunk expressways linking Tianjin with Beijing and several major cities in the northern provinces of Hebei and Henan.

Traffic jams clogged downtown streets and rural roads for about four hours in the morning, causing a series of car pile-ups. No casualties have been reported so far.

According to observatory experts, the fog was caused by a combination of high humidity and lower temperatures. They predicted the fog would ease on Thursday afternoon.

(Xinhua News Agency December 20, 2007)