After weeks of threatening rhetoric from the North, South Korea on Thursday promised its own unspecified “grave measures” if Pyongyang rejects talks on a jointly run factory park shuttered for nearly a month.

The park in the North Korean border town of Kaesong is the most significant casualty so far in the recent deterioration of relations between the Koreas. Pyongyang barred South Korean managers and cargo from entering North Korea earlier this month, then recalled the 53,000 North Koreans who worked on the assembly lines.

South Korea’s Unification Ministry on Thursday proposed working-level talks on Kaesong and urged the North to respond by noon today, 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 say what those measures might be. Some analysts said Seoul would likely pull out the roughly 175 South Korean managers who remain at the complex.

Kim said South Korea set a deadline because the remaining workers at Kaesong are running short of food and medicine. 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.

North Korea didn’t immediately respond Thursday, according to the Unification Ministry. North Korean state television showed fighter jets streaking across the sky and goose-stepping soldiers parading in front of leader Kim Jong Un at a ceremony in Pyongyang marking the 81st anniversary of the founding of the military. Tens of thousands of people visited Pyongyang’s Kumsusan Palace to celebrate the anniversary.

The demand for talks 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. Its demands, including dismantling all U.S. nuclear weapons, go far beyond what its adversaries will accept, but Washington, Seoul and Beijing have also pushed for an easing of animosity.

The Kaesong complex is the last major symbol of cooperation remaining from an earlier era that saw the Koreas set up various projects to facilitate better ties.

The factory park 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.