South Korea expressed regret Sunday over North Korea's move to bar border crossings by workers from a joint industrial park in the North.

Pyongyang first closed the border on March 9 after cutting off the only remaining hot line with the South to protest its ongoing military drills with the United States. The North says the exercises are a rehearsal for an invasion. The two Koreas use the hot line to coordinate the passage of people and goods through their heavily fortified border.

The North reopened the border Tuesday but closed it again Friday, stranding hundreds of people working in the Kaesong complex.

The North's move is "very regrettable," Unification Minister Hyun In-taek said at a meeting with South Korean business owners who run factories in the sprawling complex.

South Korea's ruling Grand National Party, meanwhile, urged the North to end the ban.

The two Koreas are still technically at war because their 1950-53 Korean War ended with an armistice, not a peace treaty. Tension on the peninsula has intensified in recent weeks after the North announced plans to launch a satellite, which many regional powers suspect is cover for a test of a long-range missile technology.

The border restrictions have caused jitters among South Korean business owners at the complex.

"I have not decided whether I should build more factories ... as the situation keeps deteriorating," said Yoo Byeong-gi, head of television parts maker BK Electronics.

The complex combines Seoul's technology and management expertise with Pyongyang's cheap labor. It has been a key source of much-needed hard currency for the impoverished North. More than 100 South Korean factories in Kaesong employ about 38,000 North Korean workers.

Nearly 730 South Koreans were stuck in the Kaesong complex Sunday but they were all believed to be safe, according to Seoul's Unification Ministry.

The North allowed six people — one Australian, three Chinese and two South Koreans — to return to South Korea on Saturday, said ministry spokeswoman Lee Jong-joo.

Meanwhile, North Korean leader Kim Jong Il inspected a military unit and watched its firing exercise, the official Korean Central News Agency said Sunday without specifying when and where.