Story highlights US, South Korean destroyers have Aegis missiles defense system

Experts question effectiveness of shows of force

(CNN) The US and South Korean navies put on a show of force in the Sea of Japan on Monday, the latest in a string of displays of military might in response to North Korea's testing of nuclear warheads and ballistic missiles.

Monday's exercises saw the US guided missile destroyer USS Spruance join ships, submarines and planes from the South Korean navy in waters east of the Korean Peninsula.

But during the joint drill, a South Korean helicopter with three people on board crashed into the sea, according to South Korea's Navy.

Shortly after taking off, the helicopter issued a distress signal, before disappearing from the radar, added the statement. Nearby forces have been deployed to find the downed helicopter.

It was the first time U.S. and South Korean vessels have operated together in waters so close to the east coast of North Korea, a US Navy official said. The ships were armed with long-range, precision, land-attack cruise missiles, the official said.

The drills saw the US guided missile destroyer USS Spruance join ships, submarines and planes from the South Korean navy in waters east of the Korean Peninsula.