Security experts worry about the safety of South Korean staff at the joint-Korean Kaesong Industrial Complex. Their fears are prompted by increasingly hysterical rhetoric from North Korea in the wake of tougher international sanctions over its latest nuclear test.

According to the Unification Ministry, 766 South Korean nationals entered Kaesong through the Dorasan immigration office in Paju, Gyeonggi Province on Monday morning, and 281 came out, while 512 cars went into the complex and 231 returned.

An average of 800 to 900 South Koreans are in Kaesong on a daily basis, and North Korea could hold them hostage if it wanted to.

Staff of South Korean firms in the Kaesong Industrial Complex go through the Dorasan immigration office in Paju, Gyeonggi Province on Monday.

When the North sank the Navy corvette Cheonan in March 2010, then-commander of the U.S. Forces Korea Walter Sharp expressed concerns that South Koreans working in Kaesong could be held hostage in case the conflict escalated. He told a high-ranking South Korean government official that he had trouble sleeping because of the Kaesong issue, and that withdrawing South Koreans from Kaesong should be considered.

South Korean and U.S. forces carried out joint simulated training to rescue people from Kaesong during an annual military exercise in August 2010.