Seoul, May 22 : South Korean President Yoon Suk-yeol and his visiting US counterpart Joe Biden visited an Air Force operations centre on Sunday on the last stop of the latter's three-day trip to Seoul. The two visited the Korean Air and Space Operations Centre at Osan Air Base in Pyeongtaek, 70 km south of Seoul, and met American and South Korean troops stationed there, reports Yo. The visit underscored the allies' firm readiness against North Korea's nuclear and missile threats after Yoon and Biden agreed during Saturday's summit to expand the scope of combined military exercises. Biden is the first US president to visit the centre, which is situated in an underground bunker and serves as a key command post for air and space operations, while Yoon is the first South Korean president to do so in 13 years. Before heading to the air base, Biden met Hyundai Motor Group Chair Euisun Chung, who announced plans to invest an additional $5 billion in the US for robotics and autonomous driving software development. Biden is set to head to Japan on Sunday afternoon. /IANS