South Korea launched a military drill Monday with the United States against a simulated North Korean invasion, even as tensions between Seoul and Pyongyang eased after a series of breakthroughs. Defence Ministry officials confirmed the exercise was underway from early Monday morning. Although the annual Ulchi Freedom Guardian drill is largely played out on computers, it involves more than 80,000 South Korean and US troops and has repeatedly been condemned by Pyongyang as a provocative war rehearsal. This year, however, the criticism from the North has been relatively muted as both Koreas have focused on reopening a joint industrial zone that was closed in April at the height of a surge in military tensions on the divided peninsula. After seven rounds of negotiations, the two rivals agreed last week on a framework for resuming operations at the Kaesong zone, which is an important hard currency earner for the impoverished North. Building on that breakthrough, South Korean President Park Geun-Hye urged Pyongyang to \"open its heart\" and resume reunions -- suspended three years ago -- for families separated since the 1950-53 war. The North on Sunday agreed to hold talks on the issue and also proposed re-starting South Korean tours to its Mount Kumgang resort.