This is the second time North Korea shut down borders to Kaesong commuters. The last time was in March 2009, also in protest to the annual U.S-South Korean military drills. Hundreds of South Korean workers were stranded in the park for four days with dwindling supplies that could not be restocked because of the blockade. They were allowed to return home as soon as the military exercises were over.
The South Korean government expressed "deep regret" to the entry ban and urged it to be lifted immediately.
"In order to promote investment in North Korea as North Korea itself hopes, it is necessary for North Korea to build trust not only with the South, but also with the international community," Kim Hyung-suk, spokesman for South Korea's Ministry of Unification, said.
In an official statement, the ministry pointed out that North Korea should be "more predictable" in doing business to attract investments from abroad and warned the second ban today will have "negative repercussions."
ABC News' Cho Long Park and Joanne Kim contributed to this report.