SINGAPORE - Three of the people who were on board the tugboat that sank in China's Yangtze river during a trial voyage are employees of Sembcorp Marine's tug towing subsidiary, Jurong Marine Services.

SembMarine said in a statement that the trio were witnessing a sea trial off Jingjiang, China of a new tug boat it had planned to charter.

A search and rescue mission is underway for all onboard, including the three employees, two of whom are Singaporeans and the third, an Indonesian.

SembMarine said it had informed their families and has made arrangements for them to travel to Shanghai.

"Management will continue to provide support and comfort and updates on the search mission to the families," it said.

Earlier, the Ministry of Foreign Affairs (MFA) said that four Singaporeans were onboard the Singapore-registered tugboat, JMS Delta, which was said to be carrying as many as 25 people and was undergoing testing in the eastern province of Jiangsu.

According to Xinhua, three people have been rescued so far, and search and rescue operations are ongoing.

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.