TAIPEI (Taiwan News) — Three Navy officers were missing after they were swept off a submarine near Kaohsiung City, the military said Thursday (Dec. 21).

During a routine mission off Zuoying Navy Base, equipment fell off the Dutch-built Sea Tiger, one of the older submarines operated by Taiwan’s Navy. Six officers went to repair to problem, but strong waves pulled them off the vessel.

All officers were wearing life jackets at the time, according to the Navy. Three were immediately rescued, but the three others were still missing hours later, the Liberty Times reported.

A Sikorsky S-70C anti-submarine helicopter and the Coast Guard armed patrol vessel Cijin joined civilian vessels in the search for the three officers.

The missing men were named as Sergeant Lin (林), and petty officers Yen (顏) and Chang (張). The three officers who had been rescued and were reportedly conscious, were Lieutenant Commander Chen (陳), Lieutenant Tsung (叢), and petty officer Lee (李).

The Sea Tiger is one of two Chien Lung-class submarines built in the Netherlands and has been serving in Taiwan’s Navy since 1987. The country unveiled its first indigenous submarine, the Narwhal, in September as part of an effort to cut reliance on overseas defense suppliers amid the rising threat from China.