North Korea held back seven of 50 remaining South Koreans at the suspended jointly run factory zone north of the heavily armed border on Monday, citing last-minute checks on taxes and wages.

The North withdrew its 53,000 workers from the complex this month amid spiraling tension between the two Koreas. The North had prevented South Korean workers and supplies from getting in to the zone since April 3.

The two Koreas remain technically at war under a mere truce that ended hostilities in their 1950-53 conflict and North Korea, angry at U.N. sanctions and joint South Korean-U.S. military drills, had in recent weeks threatened both countries with nuclear attack.

South Korean officials said the seven were not at risk. But the delay was another complication in what had been a decade of experiment merging the affluent South's capital and the North's cheap labor as a trial on commercial cooperation. "As the North has said in their statements about ensuring the safety of the people who will be returning, we do not consider them at risk but we will be closely watching until all of them are back," a South Korean government official said.

The Kaesong project opened in 2004 as part of the so-called sunshine policy of engagement and optimism between the two Koreas, and 123 South Korean companies produced clothing, household goods and motorcycle helmets employing local workers.

The zone was a lucrative source of cash for the North, providing it with almost $90 million a year. South Korean manufacturers have been paying about $130 a month to North Korea for each of the workers they employed.