South Korea said Thursday that it has decided to partially limit the entry of its citizens into a joint industrial park in North Korea following the North's claimed successful test of a hydrogen bomb.

The Unification Ministry said that it will only permit South Korean businessmen directly involved in the operation of the factories at Kaesong Industrial Complex to enter the park "for the time being." It did not elaborate on how long its measure will be effective.

"The government sees the inter-Korean situation sparked by the North's nuclear test as very grave. We will take necessary steps to ensure the safety of our nationals at most," said a ministry official, asking not to be named.

North Korea said Wednesday that it has succeeded in conducting an H-bomb test, claiming that it has become a nuclear state that even possesses an H-bomb. But outside experts at home and abroad voiced doubts about the North's assertion.

If confirmed, it will be the North's fourth nuclear test. The North conducted nuclear tests in 2006, 2009 and 2013.

The complex in the North's border city of Kaesong opened in 2004 as a symbol of inter-Korean reconciliation. A total of 124 South Korean firms are running factories with about 54,000 North Koreans working in them.

The ministry official said that if the entry restriction takes effect, the number of South Koreans staying at the factory zone could fall by up to about 100 per day.

As of Thursday morning, about 1,200 South Koreans were staying at the complex for business activities.

South Korea took a similar step in August, when inter-Korean tension heightened following the North's firing of shells at a South Korean front-line military unit. The restriction was lifted after the two sides reached a rare deal on Aug. 25 to defuse military tension.

The official said that South Korea took a similar step in 2013, when the North conducted its third nuclear test.

"The North's latest test can affect the operation of the factory zone," the official said. He also added that it is too early to predict whether the Kaesong complex can be subject to the U.N. Security Council's fresh sanctions against the North.

The industrial complex has served as a major revenue source for the cash-strapped North, while South Korea has benefited from cheap but skilled North Korean labor. (Yonhap)