South Korea said Thursday it was imposing partial restrictions on entry to a joint industrial complex in North Korea, a day after the hermit kingdom shocked the world with its fourth nuclear test.

The Unification Ministry said it will only permit South Korean businessmen and those directly involved in the operation of the Kaesong Industrial Complex to cross the border for the time being for safety concerns.

"The entry restriction is a measure to ensure the safety of the citizens in this state of emergency," a ministry official told AFP, asking not to be named.

Around 500 South Koreans still crossed the border to Kaesong on Thursday, but the official said the number would quickly be reduced.

The move was described as "an initial countermeasure", with the official suggesting further Kaesong-related restrictions could be imposed in the future.

"Once we get the full picture of international sanctions on North Korea, the measure will need to be reviewed," the official said.

With backing from China, Pyongyang's sole major ally, the 15-member UN Security Council on Wednesday strongly condemned the test and said it would begin work on a new UN draft resolution that would contain "further significant measures".

The Kaesong industrial estate opened in 2004 and currently hosts more than 120 South Korean companies which employ some 53,000 North Korean workers.

The estate is a precious source of hard currency for the isolated and impoverished North. The South Korean firms get cheap labour as well as preferential loans and tax breaks from the government.

The business park -- virtually the last remaining form of economic cooperation between the Koreas -- has become increasingly vulnerable to turbulent swings in inter-Korea politics.

In 2013, during a period of heightened cross-border tensions, Pyongyang effectively shut down the zone for five months by withdrawing its workers.