By Tuesday afternoon, only 14 confirmed survivors had been rescued and seven wer confirmed dead by Wednesday morning, prompting grave fears for those remained trapped inside the ship, which overturned on Monday night. Part of the hull was still visible above the river's  surface.

The accident is likely to end up as China's worst shipping disaster in almost 70 years.

Weather has so far been the only cause raised for the vessel's capsizing, with Xinhua news agency citing the ship's captain and chief engineer, who were rescued, as saying that a tornado had stuck them. China's Meteorological Administration confirmed on Tuesday that a tornado, with level 12 winds on the Beaufort Wind scale, had hit the area at the same time as when the ship capsized, according to Xinhua.

China's state broadcaster, China Central Television, showed dramatic pictures of rescue workers who had clambered onto the stricken vessel's hull, and said they could hear the sounds of people trapped inside.

On board the ship were 406 tourists, most aged between 50 and 80, on a tour organised by a Shanghai tour group, according to local media reports. There were 47 crew members on board, as well as five tour guides. Distraught family members were seen gathering outside the tour company's offices, anxious for news.

One of the people pulled from the capsized boat was a 65-year-old woman. Divers fixed breathing equipment to her nose and mouth to bring her up from under the water.

About another dozen people had been rescued and six bodies recovered, media reported, leaving more than 430 people unaccounted for.

Tour guide Zhang Hui, 43, told Xinhua the boat sank very fast and he scrambled out a window in torrential rain, clutching a life vest as he could not swim.

"Wave after wave crashed over me; I swallowed a lot of water," Zhang said, adding that he was unable to flag down passing boats and finally struggled ashore as dawn broke holding onto a branch.

Among the confirmed survivors were the ship's captain and chief engineer, both taken into police custody, although it was not immediately clear if this was merely routine procedure.

The ship's captain reportedly said the ship was hit by cyclonic winds. No distress call was made and the vessel capsized within two minutes in 15-metre deep waters.

China's weather bureau said a tornado hit the area where the boat was, a freak occurrence in a country where tornados do happen but are not common.

The ship  was heading to south-west China's Chongqing city from Nanjing, capital of east China's Jiangsu province, a popular scenic journey which takes several days. It sank about  9.28pm  in the Jianli section of the river, in Hubei province.

The Hubei Daily said the ship, the Eastern Star, was owned by the Chongqing Eastern Shipping Corporation. The company has been operating since 1967, it said, and the ship was capable of carrying 534 people.

Prime Minister Li Keqiang arrived at the scene on Tuesday, Xinhua reported. The official news agency also said Xi Jinping, the country's president, had "issued important instructions immediately" to direct rescue operations, an indication of how seriously the party regarded the accident.

More than 3000 rescuers, including soldiers, policemen and civilians, as well as 150 ships were taking part in the rescue operation, according to the China Daily.

The Yangtze is the world's third-longest river and one of China's most important waterways, with the Three Gorges area particularly scenic and popular with tourists, both from overseas and within mainland China.

Accidents of this magnitude are uncommon in China where major rivers are used for tours and cruises. A tug sank on the Yangtze while undergoing sea trials in January, killing 22 of 25 people on board.

In the worst previous incident of its kind in China, the steamship Kiangya blew up on the Huangpu river in 1948, killing more than 1000 people.

With agencies, Bloomberg

Updated: June 3, 12:24pm

Follow FairfaxForeign on Twitter

Follow FairfaxForeign on Facebook