Most remaining South Korean workers at a troubled joint industrial zone in North Korea returned home early on Tuesday after Seoul ordered a pullout following months of tensions.

Forty-three workers from Kaesong -- once a rare symbol of inter-Korean cooperation -- crossed back over the world's most heavily militarised frontier shortly after midnight, according to Seoul's Unification Ministry.

But seven supervisors remained for talks with the North Koreans about unresolved administrative issues such as the wages of local workers, according to the ministry, which did not say when they were expected to return.

The evacuation raises the prospect of the permanent closure of the industrial park, the last point of contact between the two Koreas and a key source of income for Kim Jong-Un's isolated regime.

South Korean companies with factories at the site have expressed shock at the sudden pullout.

The complex is the victim of escalating tensions triggered by a nuclear test by the North in February, which has been followed by a series of bellicose threats of nuclear war and missile tests by Kim Jong-Un's regime.

The South's Unification Ministry played down concerns about the seven remaining employees, who work for the Kaesong Industrial District Management Committee and telecoms company KT Corp.

"We believe there is no chance of South Korean officials being held as hostages because both sides have been locked in talks on specific and practical issues since Friday," said a ministry official.

"North Korea did not raise such issues suddenly to block them from crossing the border. We hope both sides will narrow their differences soon."

Seoul announced on Friday that it had decided to pull all employees from Kaesong after Pyongyang blocked access to the site and refused to open talks on restarting the stalled operations.

South Korean Foreign Minister Yun Byung-Se told a forum in Seoul on Monday that "the window of dialogue is still open" on Kaesong, according to the South's Yonhap news agency.

But some observers believe the shutting down of the complex would be permanent as the factory equipment there would fall into disrepair and the firms would soon lose their customers.

"Some people say that the complex may be reopened in a few weeks or months once the two sides hammer out a deal, but it's a ludicrous idea," Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul, told AFP.

"Once the complex dies, the North will naturally deploy its troops back there, returning the military situation to the pre-Kaesong days. All the artillery units targeting Seoul will move closer to the border, which will surely heighten military tension."

Pyongyang announced on April 8 that it was pulling out its 53,000-strong workforce from Kaesong, angered by the South's mention of a "military" contingency plan to protect its staff at the site.

Established in 2004, the complex lies 10 kilometres (six miles) inside the North, which remains technically at war with the South after the 1950-53 Korean War was concluded with a ceasefire rather than a peace treaty.

The North on Monday renewed its own threat of "final decisive" action on Kaesong if the situation worsens.

The pullout is "a cunning and mean-spirited trick aimed at passing blame to the North for the plight of the complex", said an editorial in the North's ruling communist party newspaper, the Rodong Sinmun.

Yonhap reported on Sunday that North Korea appeared to be preparing for a major live-fire military exercise on its west coast involving artillery units and air force jets.

Joint South Korea-US military drills which have angered the North ended on Tuesday.