SEOUL -- The Democratic People's Republic of Korea (DPRK) banned South Korean workers' entrance to the joint industrial complex at the DPRK's border town of Kaesong, only allowing the workers to leave Kaesong to come back to Seoul, the unification ministry said Wednesday.

"In the morning, North Korea (DPRK) notified us of the ban of entrance to the Kaesong industrial park, while only allowing for leaving Kaesong," Kim Hyung-seok, spokesman of the Ministry of Unification in charge of dialogue and cooperation with the DPRK, told reporters at a press conference.

Kim said that the South Korean government "deeply regrets the failure of normal cross-border movement" in and out of the inter- Korean industrial zone, noting that it will put its top priority at ensuring the safety of workers at the industrial site.

The entry ban came four days after Pyongyang threatened to shut down the joint industrial park in the DPRK border town of Kaesong, which is housing around 120 South Korean companies and is employing some 54,000 DPRK workers.

Despite the normal operation of factories at the inter-Korean industrial zone until now, the entry ban would stop the supply of additional raw material and workers to the plants, which may lead to a disruption in production if the ban is prolonged.

The DPRK said on Tuesday that it will restart the five-megawatt graphite moderated reactor, which was shut down in October 2007 under an agreement at the six-party talks. A spokesman of the DPRK 's department for atomic energy said the restart will be made " without delay".

Tensions escalated on the Korean Peninsula after Pyongyang said last weekend that it had entered "a state of war" against Seoul. The DPRK warned that it will nullify the Armistice Agreement that ended the 1950-53 Korean War, vowing to nullify all non-aggression treaties. It cut off all cross-border hotlines.

The recent series of threats came in protest against the military drills between Seoul and Washington, including the two- month field trading exercise "Foal Eagle" and the computer- simulated war game "Key Resolve".