President Yoon Suk Yeol returned home Friday from his five-day trip to the United States that was focused on bolstering security against rising threats posed by deepening military ties between North Korea and Russia.Yoon attended the North Atlantic Treaty Organization summit in Washington on Wednesday and Thursday as South Korea was invited to the summit for the third consecutive year as one of four Indo-Pacific partners alongside Japan, Australia and New Zealand.During the main session and a series of bilateral and multilateral meetings, Yoon expressed concerns over North Korea's forging of a sweeping mutual defense pact with Russia last month, stressing the interlinked security between Europe and Asia.On Thursday, Yoon held a bilateral meeting with U.S. President Joe Biden on the sidelines of the summit to adopt a statement on updating the guidelines for the Nuclear Consultative Group, which outlines the allies' planning and coordination of a joint nuclear response in case of North Korea's nuclear at tacks.Ahead of the summit, Yoon visited Hawaii on Monday and Tuesday to meet U.S. military commanders at the Indo-Pacific Command, which oversees the U.S. Forces Korea and other key strategic posts in the Indo-Pacific region.Source: Yonhap News Agency