Three people were missing after a South Korean naval helicopter crashed into the sea off the country's east coast Monday night during a joint military drill with the US, the defence ministry said.

The Lynx helicopter sent out a distress signal and disappeared from radar shortly after taking off from a warship on the East Sea (Sea of Japan) at around 1200 GMT, a ministry spokesman said.

The aircraft apparently crashed into the sea, he said, adding search efforts were underway to find the three who were on board, including the pilot.

Monday's joint naval exercise was the latest display of military might in reponse to North Korea's fifth nuclear test earlier this month, which raised tension on the peninsula.

It involved US guided-missile destroyer USS Spruance and South Korean warships, and simulated attacks on the North's troops as well as submarine detention, Yonhap news agency said.