North Korea has closed the inter-Korean border three days after reopening it to South Koreans working at a joint industrial complex in the North.

The latest move on Friday prevented some 600 South Korean workers from traveling to the Gaeseong Industrial Complex and 250 from returning home from the North's border city.

The Unification Ministry in Seoul said that the group including two Chinese and one Australian was supposed to return to the South later in the day.

North Korea refused without reason to issue permission for the cross-border passage. The ministry expressed deep regret over the border closure, urging the North to allow the South Korean workers to travel to and from Gaeseong.

The North's closure of the inter-Korean border --- the second in three days --- is raising concern over the safety of South Korean nationals working in the Gaeseong complex.

On Monday, North Korea cut inter-Korean military hotlines and blocked the border traffic in protest of the annual South Korea-U.S. joint military exercises, but lifted the travel ban the following day.

Currently, some 760 South Koreans are staying in the North --- some 730 at the Gaeseong complex and 30 at the Mount Geumgang resort near the east coast.