The government says it is strengthening measures to beef up security for South Koreans staying in North Korea amid reports that Seoul has reached a conclusion that the “Cheonan” naval vessel was sunk by a North Korean torpedo.

The Unification Ministry in Seoul on Wednesday told a regular news briefing that South Korean firms operating in North Korean areas other than the Gaeseong Industrial Complex are pulling out their employees.

The ministry said South Korean firms that have collected sand in the North's Haeju and Goseong areas have sent all their sand transport vessels back to the South.

The ministry also asked South Koreans at the North's Mount Geumgang resort to refrain from making unnecessary contact with North Koreans and to use prudence in speech and action.