South Korea's defence ministry said Wednesday it had contingency plans, including possible military action, to ensure the safety of its citizens working in a joint industrial zone in North Korea. "We have prepared a contingency plan, including possible military action, in case of a serious situation," Defence Minister Kim Kwan-Jin told ruling party MPs in a meeting. "We should try to prevent the situation from going to the worst," Kim added. North Korea blocked South Korean access to the Seoul-funded Kaesong joint industrial zone on Wednesday, but said it would allow the 861 South Koreans currently there to leave. As of 2:00 pm (0500 GMT) only nine had crossed back over the border into South Korea. The South's Unification Ministry said many had voluntarily opted to stay in Kaesong to ensure the smooth operation of their companies there. Meanwhile, a Chinese official met ambassadors from the United States and both Koreas to express "serious concern" about the situation on the Korean peninsula, China's Foreign Ministry said on Wednesday after North Korea closed access to the joint factory zone with the South. China hopes North Korea and South Korea can resolve their differences through talks, Foreign Ministry spokesman Hong Lei told reporters at a daily news briefing. Factories in the Kaesong Industrial Park were still believed to be operating, but North Korea's decision to block entry is a further sign of the growing tensions on the Korean peninsula. On Tuesday, Pyongyang said it would restart a mothballed nuclear reactor.