Rescuers Wednesday started cutting through the hull of a capsized Chinese cruise ship, state media said, in a desperate effort to find survivors among more than 400 people still missing days after the disaster.

Only 14 people have been found, along with 26 bodies, since the "Eastern Star" overturned late Monday in a storm on the Yangtze river, leaving part of the boat protruding from the murky water.

Chinese state media agency Xinhua warned the disaster could become the deadliest in the country for almost seven decades as rescue workers started cutting a small hole in the bottom of the ship in an attempt to free anyone still inside.

"The ship sank in a very short timeframe so there could still be air trapped in the hull," it quoted Li Qixiu of the Naval University of Engineering as saying, which "means there could still be survivors."

Li said divers had attached steel cables to the hull and the plan was to support the body of the boat with cranes while rescuers searched inside.

It would be important to hold the ship steady to stop it sinking further during the delicate operation, he told Xinhua.

Witnesses and state media said the ship -- which was carrying a total of 456 people, most aged over 60, on a holiday cruise -- sank in a matter of seconds after it was hit by bad weather. Weathermen said a tornado was in the area at the time.

Around 20 bodies appeared to have been found Wednesday with dozens of divers searching each of the ship's cabins one by one, CCTV said.

A transport ministry spokesman told AFP that rescuers were battling low visibility in the muddy waters, but would keep searching even as hopes of finding survivors dwindled.

Chinese Premier Li Keqiang, who has been directing operations, vowed to keep searching despite the tough conditions.

"As long as there is the slightest hope, we must go all-out to find the missing," Xinhua quoted him saying.

- Relatives clash with police -

Information on the disaster has been strictly controlled and officials gave little away during a news conference on Wednesday afternoon, providing no figures on deaths or survivors and taking no questions.

Relatives of the passengers clashed with police earlier Wednesday as hope that survivors would be found turned to anger at a lack of news.

A video shared on social media showed pushing and shoving between police and angry relatives outside a local government building in China's commercial hub of Shanghai, where many of the passengers hailed from.

"The police first formed a human wall and didn't let us in. Then the relatives got excited and started to shout. Some policemen hit people," said one young woman whose mother was on the boat.

The mother of seven-year-old Yang Chenlin, who was on the boat with her grandparents, said relatives were desperate for more information. "We need to go to the site. That's our common appeal," she said.

At the Jianli County People's Hospital, frantic relatives arrived searching for missing loved ones.

"We drove from 10 pm last night to 6 am this morning to get here," a woman who looked pale and wan told AFP at the hospital, adding that her uncle and aunt had been on board.

"We don't really know anything," said a man who had travelled with her.

Hospital officials by late Wednesday had sealed off the facility from reporters, along with a local funeral parlour.

- Tight media control -

China tightly controls its domestic media and a government directive posted online by the US-based China Digital Times said local outlets had been ordered to only use reports from state media.

Roadblocks were sited about two kilometres (1.2 miles) from the capsized vessel, with cars being turned back even before that point. Authorities limited access for foreign journalists to a brief river trip.

At the site, CCTV showed Li bowing in the rain to bodies covered in white sheets.

It said the 250-foot (76.5-metre) vessel had floated three kilometres downriver after it capsized.

Zhang Hui, a 43-year-old tour guide on board who survived despite being unable to swim, told Xinhua he had just "30 seconds to grab a life jacket".

The captain and chief engineer, who were also among the survivors and were being questioned by police, both reportedly said the ship was caught in a freak storm.

The ship was cited for safety infractions two years ago along with five other vessels, according to a notice by the Nanjing Maritime Bureau, which gave no details on why the boat was detained or subsequent actions taken.

Condolences for the disaster came from the United States, European Union, the UN, Australia and the Vatican.

"I wish to express my closeness to the Chinese people in these difficult moments after the ferry disaster in the Yangtze River," said Pope Francis.