North Korea conducted a big live-fire exercise on Tuesday to mark the foundation of its military as a U.S. submarine docked in South Korea in a show of force amid growing concern over the North's nuclear and missile programs. The nuclear-powered submarine the USS Michigan, which arrived in the South Korean port of Busan, is built to carry and launch ballistic missiles and Tomahawk cruise missiles.The U.S. 7th Fleet says two of its destroyers are conducting simultaneous maritime exercises with naval ships from Japan and South Korea. The military exercises came amid heightened concerns over a possible nuclear or missile test by North Korea as it marks the 85th anniversary of its army.The U.S. Navy said the USS Wayne E. Meyer and the South Korean navy's Wang Geon are conducting exercises in waters west of Korean Peninsula. It said the USS Fitzgerald is teaming up with the Japanese destroyer Chokai in waters west of Japan.It said the exercises demonstrate a shared commitment to security and stability in the region and underscore America's flexibility in combining with allied naval forces "in response to a broad range of situations."Around 30,000 American troops are stationed in South Korea as part of a military strategy that Pyongyang condemns as a preparation for war.A U.N. human rights expert is calling for a cooling of tensions on the Korean Peninsula, insisting that statements that feed hatred and polarization undermine the chance to "improve the dire situation of ordinary North Koreans."Tomas Ojea Quintana, special rapporteur on North Korea for the U.N. human rights office, says "the recent rise in conflict rhetoric is worsening already critical human rights challenges in North Korea." He said the world should come together to protect the rights of people in North Korea.Ojea noted increased tensions caused by North Korea's nuclear and missile tests and the deployment of a U.S. aircraft carrier group in the region. He said "the onus is on all of us to lower tensions and restore dialogue, including on human rights."North Korea's growing nuclear and missile threat is perhaps the most serious security challenge confronting U.S. President Donald Trump. He has vowed to prevent North Korea from being able to hit the United States with a nuclear missile and has said all options are on the table, including a military strike. Trump sent the USS Carl Vinson aircraft carrier strike group for exercises off the Korean peninsula as a warning to North Korea and a show of solidarity with U.S. allies.South Korea's navy said it was conducting a live-fire exercise with U.S. destroyers in waters west of the Korean peninsula and would soon join the carrier strike group approaching the region.Matching the flurry of diplomatic and military activity in Asia, the State Department in Washington said on Monday U.S. Secretary of State Rex Tillerson would chair a special ministerial meeting of the U.N. Security Council on North Korea on Friday.Tillerson, along with Defense Secretary Jim Mattis, Director of National Intelligence Dan Coats and Joint Chiefs chairman General Joseph Dunford, would also hold a rare briefing for the entire U.S. Senate on North Korea today, Senate aides said.