Sea ice off China's east coast is killing fish stocks and causing huge losses to the fishing industry -- and the ice is set to worsen, the State Oceanic Administration warned Friday.

Almost 30,000 square km of sea ice was covering up to 40 percent of the Bohai Sea off east China's Shandong Province, said Lin Shanqing, director of disaster relief department of the State Oceanic Administration.

Lin said sea ice in Liaodong Bay had expanded to almost 70 nautical miles off the coast Friday from 60 nautical miles on Tuesday. Sea ice in Laizhou Bay and Bohai Bay both expanded to 30 nautical miles Friday from 20 nautical miles on Sunday.

The worst sea ice in 40 years appeared began early January along the coasts of the Bohai Sea and the Yellow Sea as cold fronts pushed temperatures below minus 10 degrees Celsius, according to the National Marine Forecasting Station on Sunday.

Lin said the severe conditions would not improve in the next week.

Sea ice had affected fisheries of Shandong's Qinghai, Yantai, Weihai port cities, as stocks died and boats were trapped in port.

The direct economic losses of the fishing industry in Weihai City was estimated at 420 million yuan (61 million U.S.dollars), according to the city's ocean and fishery bureau.

In Yantai City, 200 boats were trapped in Haimiao port, Laizhou Bay, for more than a month.

An oil tanker was trapped in ice in the Bohai Sea Thursday, but was towed to port without casualties or oil leakage.

Shandong Provincial oceanic and fishery department had issued warnings of sea ice and began 24-hour monitoring of conditions Tuesday.