North Korea said it will use force immediately if South Korean ships cross into its territorial waters. Pyongyang issued the statement following the start of Seoul's war games in the Yellow Sea. Pyongyang is also threatening to close access to industrial facilities where more than 700 people from South Korea are working. The two Korea's found themselves on the brink of the war after March 26, when a South Korean corvette Cheonan was sunk with 46 sailors on board. The investigation stated that it was a North Korean torpedo which sank the ship. Pyongyang denies it. A group of Russian experts is set to go to Seoul to conduct an alternative investigation.