South Korea said Monday it will further limit access by its nationals to a joint industrial park in North Korea to ensure their safety following the North's claimed test of a hydrogen bomb last week.

The Unification Ministry said that it plans to only permit entry by a "minimum" number of South Koreans to the Kaesong Industrial Complex in the North's border city of the same name. The move will be effective starting Tuesday.

A total of 124 South Korean firms are running factories with about 54,000 North Koreans working at the complex, which opened in 2004 as a symbol of inter-Korean reconciliation.

Last week Seoul began restricting the entry of its nationals into the complex on concerns that heightened geopolitical tension may threaten their safety.

The government said Monday that it will only allow South Korean businessmen directly involved in the operation of factories to enter the park. Contractors will only be allowed into the park if they enter and leave on the same day, it said.

"The move is aimed at securing the safety of South Koreans as the North is expected to react to Seoul's resumption of anti-North Korean loudspeaker broadcasts," Jeong Joon-hee, ministry spokesman, told a regular press briefing.

The decision will likely reduce the number of South Koreans staying there from around 800 per day to some 650, he added.

"Whether to apply further measures will depend upon relevant circumstances," Jeong added.

North Korea claimed to have successfully conducted its first hydrogen bomb test, saying that the move is an act of self-defense in the face of nuclear war threats by the United States.

Seoul has vowed to make the North "pay the price" for its nuclear provocation and resumed the loudspeaker broadcasts after about four months of silence, a move certain to irritate the North.

Washington's B-52 strategic bomber flew in the skies of South Korea on Sunday in a show of force. The plane was armed with nuclear missiles and "bunker buster" bombs that are capable of bombarding North Korea's underground facilities. (Yonhap)