SHANGHAI - Twenty-two people including four Singaporeans were missing after a tugboat sank on a trial voyage in China's vast Yangtze river.

The newly built vessel was undergoing testing with 25 people aboard in the eastern province of Jiangsu on Thursday afternoon when the accident happened, state media said.

Three people were rescued and 22 still missing, China Central Television reported, adding that around eight foreigners were among those missing.

Singapore's Ministry of Foreign Affairs said the tugboat is a Singapore-registered vessel, JMS Delta, and four Singaporeans are missing.

Mr Stanley Loh, Singapore's ambassador to China, told The Straits Times that officers from the Shanghai consulate-general, including the Consul-General Ong Siew Gay, are now on site providing help to the families and the companies involved.

"Search and rescue is now our top priority. I have spoken with Jiangsu vice-governor Zhang Lei, who is taking charge of rescue operations. He assured me that the Jiangsu authorities are doing everything possible to rescue the crew," said Mr Loh.

"I told him Singapore is ready to provide any necessary assistance too. Our prayers are with the crew members and their families."

The Japanese and Indian consulates in Shanghai each confirmed to AFP that one of their nationals was among the missing. A microblog posting by a newspaper under China's transport ministry said two of those on board were from Malaysia and Indonesia.

Singapore-based marine and offshore engineering group Sembcorp Marine has issued a statement on Friday saying its tug towing subsidiary Jurong Marine Services has three employees - two of whom are Singaporeans and one an Indonesian - on the boat.

"The Group has informed their families and has made arrangements for them to travel to Shanghai," said the company.

The accident occurred on a stretch of the river between the cities of Jingjiang and Zhangjiagang, which is close to the Yangtze's mouth near the commercial hub Shanghai.

"Water entered the boat cabin very quickly, in less than 20 seconds it was completely filled with water," survivor Wang Zhenkai told state television from his hospital bed. He was accompanying a Japanese technician who was testing the engine, though the ship was made and outfitted in China, reports said.

A photo carried showed only the bow and part of the hull of the metal ship floating above the waterline, with a salvage barge alongside.

Reports said rescue workers were trying to raise the vessel and the search was continuing, but Xinhua cited rescuers as saying that the work was difficult as the current was swift and the water cold.

"As long as we have the slimmest hope, we will give a 100-per cent effort," Wang Shiming, deputy head of the Jiangsu Maritime Safety Administration, told state television.

The provincial government said the boat was undergoing trials without properly completing the required procedures and without first reporting the condition of the ship as required by regulations, reports said.

The operator "should have reported to the responsible government body for endorsement, but did not," Wang said.

(Additional reporting by Kor Kian Beng, with inputs from AFP, Xinhua)