SHENYANG -- Rapidly expanding sea ice has interrupted offshore drilling, fishing and other economic activities on the Bohai and Yellow seas.

The Meteorological Bureau in China's northeast Liaoning Province said Friday 9,795 square km of ice had developed on the two seas.

Ice extending out from Liaodong Bay -- in the north of the Bohai Sea -- was 3,191 square km larger than on January 1, and ice in the Yellow Sea expanded by 406.6 square km during the same period, according to the bureau's figures.

The average temperature of minus 12.5 degrees Celsius from Monday to Wednesday for the Bohai Sea was 2.3 degrees lower than normal.

Beihai Forecasting Center of State Oceanic Administration said Friday the average sea ice thickness in the Bohai and Yellow seas was between 5 and 15 cm.

Ice breakers are working to keep all major ports in north China open.

However, a gas station worker surnamed Zhao at the port of Tianjin said few transportation vessels had come to refuel recently because of the sea ice.

More than 500 fishing boats are docked in Tianjin. Fishermen said the sea ice normally thaws in March.