Seoul - North Korea's military on Tuesday greeted the start of annual US-South Korean military drills with its standard fiery threats, vowing "merciless retaliation" for exercises Pyongyang claims are an invasion rehearsal.North Korea routinely issues such warlike rhetoric or conducts weapons tests to respond to the US-South Korean exercises.Tuesday's threat came as top US generals, including Admiral Harry Harris, the commander of US forces in the Pacific, visited South Korea.Ties between the Koreas are almost always fraught, but anxiety is higher than normal following weeks of tit-for-tat threats between US President Donald Trump and Pyongyang in the wake of the North's two intercontinental ballistic missile tests in July.The US generals were to travel to the site of a contentious US missile-defence system in South Korea later on Tuesday.Direct attackThe North's military statement said it will launch an unspecified "merciless retaliation and unsparing punishment" on the US over the Ulchi Freedom Guardian drills that began Monday for an 11-day run.Despite the threat, an unprompted direct attack is extremely unlikely because the US vastly outguns Pyongyang, which values the continuation of its dictatorship above all else.Impoverished North Korea hates the drills in part because they force it to respond with expensive military measures of its own.The North Korean statement accused the US of deploying unspecified "lethal" weapons for the drills that it says involve a "beheading operation" training aimed at removing absolute ruler Kim Jong Un."No-one can vouch that these huge forces concentrated in South Korea will not go over to an actual war action now that the military tensions have reached an extreme pitch in the Korean peninsula," the statement read."Moreover, high-ranking bosses of the US imperialist aggressor forces flew into South Korea to hold a war confab. Such huddle is increasing the gravity of the situation."The drills are largely computer-simulated war games held every summer and have drawn furious responses from North Korea.This year's exercise involves 17 500 American troops and 50 000 South Korean soldiers, according to the US military command in South Korea and Seoul's Defence Ministry.GoalNo field training like live-fire exercises or tank manoeuvring is involved in the Ulchi drills, in which alliance officers sit at computers to practise how they would engage in battles and hone their decision-making capabilities.The allies have said the drills are defensive in nature.In July, North Korea test-launched two ICBMs at highly lofted angles and outside experts say those missiles can reach Alaska, Los Angeles or Chicago if fired at normal, flattened trajectories.Analysts say it will be only a matter of time for the North to achieve its long-stated goal of acquiring a nuclear missile that can strike anywhere in the US.Earlier in August, Trump pledged to answer North Korean aggression with "fire and fury".North Korea, for its part, threatened to launch missiles toward the American territory of Guam before Kim Jong Un said he would first watch how Washington acts before going ahead with the missile launch plan.