FILE - In this June 22, 2006 file photo, North Koreans work at a factory of South Korean apparel maker Shinwon company in the inter-Korean industrial park in Kaesong, North Korea. South Korea on Thursday, April 25, 2013 warned of an unspecified "grave measure" if North Korea rejects talks on the jointly run factory park shuttered for nearly a month - setting up the possible end of the last remaining major symbol of inter-Korean cooperation. (AP Photo/Ahn Young-joon, File) U.S. Army soldiers ride an armored vehicle during their military exercise near the border village of Panmunjom, that separates the two Koreas, in Paju, north of Seoul, South Korea, Wednesday, April 24, 2013. Washington and Seoul said they've seen no evidence that Pyongyang is actually preparing for a major conflict, though South Korean defense officials said the North appears prepared to test-fire a medium-range missile capable of reaching the American territory of Guam. SEOUL, South Korea (AP) - South Korea on Thursday warned of unspecified "grave measures" if North Korea rejects talks on a jointly run factory park shuttered for nearly a month - setting up the possible end of the last remaining major symbol of inter-Korean cooperation. The demand for talks, which is likely to draw an angry response from North Korea, follows a lull in what had been a period of rising hostility between the Koreas. Pyongyang has recently eased its threats of nuclear war and expressed some tentative signs of interest in dialogue, and Washington and Seoul have also pushed for an easing of animosity. Despite North Korea's threats, there were few major actions; perhaps the biggest was Pyongyang's suspension of operations at the inter-Korean factory park in the North Korean border town of Kaesong. Early this month, it barred South Koreans from crossing the border and entering the factory, which is a holdover from an era that saw the Koreas set up various cooperative projects meant to facilitate better ties. It also withdrew the 53,000 North Koreans who manned assembly lines there. South Korea's Unification Ministry on Thursday proposed working-level talks on Kaesong and urged the North to respond by noon Friday, warning that Seoul will take "grave measures" if Pyongyang rebuffs the call for dialogue. In a televised news conference, spokesman Kim Hyung-suk refused to describe what those measures might be, but some analysts said Seoul would likely pull out its remaining workers from the complex if the working-level talks don't happen. The factory has operated with South Korean know-how and technology and with cheap labor from North Korea since 2004. It has weathered past cycles of hostility between the rivals, including two attacks blamed on North Korea in 2010 that killed 50 South Koreans. More than 120 South Korean companies, mostly small and medium-sized apparel and electronics firms, operated at Kaesong before North Korean workers stopped showing up on April 9. Raw material came from South Korea, with finished goods later sent back south. Last year, the factories produced goods worth $470 million. South Korean companies paid salaries to North Korean workers averaging $127 a month, according to South Korea's government. That is less than one-sixteenth of the average salary of South Korean manufacturer workers. Impoverished North Korea objects to views in South Korea that the park is a source of badly-needed hard currency. It also has complained about alleged South Korean military plans in the event Pyongyang held the Kaesong managers hostage. At the factory, food has dwindled, and with no apparent end to North Korea's suspension in sight, a daily trickle of South Korean workers have returned home. Still, Pyongyang hasn't forced South Koreans to leave, and about 175 are still there. Kim, the Unification Ministry spokesman, said South Korea set a deadline because the remaining workers at Kaesong are seeing food and medicine shortages. He said the companies there are suffering economically because of the shutdown. To resolve deadlocked operations at Kaesong, Kim said North Korea should first allow some South Koreans to cross the border to hand over food and medicine to the managers. South Korea on Wednesday proposed talks, but the North rebuffed the offer, Kim said. "It's very regrettable for North Korea to reject (taking) the minimum humanitarian measures for our workers at the Kaesong industrial complex," he said.