SINGAPORE - Four Singaporeans were onboard the tugboat that sank in China's Yangtze river during a trial voyage on Thursday, the Ministry of Foreign Affairs (MFA) said.

The Singapore-registered vessel JMS Delta, which was carrying 25 people, was undergoing testing in the eastern province of Jiangsu, through which the river runs.

Three people have been rescued, the remaining 22 are missing.

An MFA spokesman said the Ministry is in touch with the families of the Singaporeans and the companies they are working for.

"Our Consulate-General officers in Shanghai are at the incident site providing consular assistance to the next-of-kin. We are also working closely with the Chinese authorities on the ongoing search and rescue operations," the spokesman added.

Besides the Singaporeans, others onboard included Indian, Indonesian, Japanese and Malaysian nationals, according to media reports.

Mr Stanley Loh, Singapore ambassador to China, told The Straits Times that the Singapore mission established contact with the Jiangsu authorities shortly after learning about the sinking of the vessel.

"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."