The US and South Korea on Monday began annual war games involving tens of thousands of troops, prompting North Korea to call its military into "full combat readiness," saying it views the joint land and sea exercises as a prelude to an invasion. The hostilities raised tensions on the Korean peninsula to their highest point in weeks as the US and its allies anxiously awaited North Korea's test launch of its most advanced long-range missile. North Korean officials late last week declared that the isolationist nation could not guarantee the safety of South Korean passenger jets flying near its airspace during the 12-day exercises. Several airlines immediately announced that they would avoid North Korean airspace as a precaution. Northern officials also cut off a military hotline, leaving North and South Korea without means of communication during the escalating brinkmanship. South Korea on Monday expressed regret over the suspension of what it called a crucial channel of contact. Southern officials said 726 of their citizens also were barred from crossing into North Korea to the jointly run Kaesong Industrial Complex. "We, the government, feel sorry that North Korea continues to take regrettable measures, although we are reacting with patience," said South Korea's Ministry of Unification spokesman Kim Ho-nyeon. US military officials here tried to diffuse tensions while stressing that the exercises, which will be larger in scale and duration than in previous years, would continue. "The primary goal is to ensure the command is ready to defend the (Republic of Korea) in the event it becomes necessary," said US Army Gen. Walter L. Sharp, commander of US forces in South Korea. He insisted that the war games were "a routine training exercise that takes place every year at about the same time. It is not tied in any way to any political or real-world event." Stephen Bosworth, the new US envoy on North Korean issues, met Monday with South Korean officials. The former US ambassador to Seoul arrived Saturday for four days of meetings at the end of an Asian tour that also took him to Beijing and Tokyo. Bosworth said he knew a tough job lay ahead. "I have no illusions about what I have agreed to do. It is a very difficult mandate," he told South Korean Foreign Minister Yu Myung-hwan, according to South Korea's Yonhap news agency. Usually testy in response to what it sees as provocations by conservative South Korean President Lee Myung-bak, North Korea proclamations have become more belligerent. Experts say North Korea is particularly incensed at suggestions made by the US and Japan that, if necessary, they could shoot down any long-range test missile launched by the North. North Korean officials in Pyongyang have said they are planning to launch a communications satellite and declared Monday that they would view any outside interference as an act of war. North Korea has the world's fifth-largest military, with, by some estimates, more than 1 million troops. The US-South Korea military exercises, called Key Resolve-Foal Eagle drills, will involve 26,000 US servicemen, including 13,100 stationed outside South Korea, and also several US destroyers and the aircraft carrier John Stennis. Experts here say verbal bombast is expected from North Korea when it feels under threat. "The US and South Korea do this military exercise annually and historically North Korea has been oversensitive," said political scientist Kim Seung-hwan, a senior associate at the Center for Strategic and International Studies. He said the impoverished nation normally resorts to words over weapons. North Korea's reported plans to test fire its Taepodong-2 missile also come at a crucial time. The last test launch in 2006 was a failure and the regime of hard-line ruler Kim Jong Il believes it needs a successful launch to boost public opinion, experts say. On Sunday, North Korea held parliamentary elections, and new members are scheduled to re-elect Kim as leader in early April. Pyongyang watchers say the government might also at that time announce the appointment of Kim's third son, Jong-un, to succeed his father, who reportedly has suffered several debilitating strokes.