Four crew members, all men, were aboard the helicopter when it crashed near the estuary of the Yangtze River at about 11:20 a.m., said a spokesman for the China Marine Search and Rescue Center.

Three were rescued and rushed to hospital, the spokesman said.

"One man is in serious condition and the other two are stable, "Zhang Guotong, president of the Shanghai Municipal No. 7 People's Hospital, told Xinhua.

More than 10 rescue boats were involved in the search for the missing mechanic, Yang Yongchang, and the helicopter, which sank, said Sun Fumin, director of the East China Sea Rescue Bureau of the Ministry of Transport.

"The water is muddy and it is getting dark, which increase difficulties for the search," Sun said. "We have to redouble the efforts and use advanced equipment."

The ship-based helicopter was rented by the Xuelong, or "Snow Dragon," which was moored at a Shanghai dock at the time of the accident. The helicopter crashed just a minute after taking off from the vessel, according to the China Marine Search and Rescue Center.

"Thick fog shrouded the sky when the crash happened," said a spokesman with the Shanghai frontier defense inspection authorities.

But the exact cause of the crash is yet to be determined.

Carrying 140 scientists, technicians and crew, the Xuelong left Shanghai in October for a 173-day trip, the longest in China's Antarctic expedition history.

The expedition team had successfully erected a new research station at Dome Argus (Dome A), the pole's highest icecap at 4,093meters above sea level.

They left China's Zhongshan Station for home on March 9 and arrived at Shanghai on April 10.

(Xinhua News Agency April 12, 2009)