Milley told his audience, "North Korea is extremely unsafe, and more risky as the weeks go by".
US media reported this week that the Defence Intelligence Agency (DIA), the Pentagon spy agency, had assessed that North Korea would be able to field a nuclear-capable ICBM by next year, earlier than previously thought.
Earlier this month Pyongyang announced it had completed its first successful test launch of an intercontinental ballistic missile, one expert said could travel far enough to reach the westernmost USA states of Alaska or Hawaii.
The official reportedly said when such equipment is seen, a launch could occur within six days.
Not long after the North's test launch, the South Korean and United States military forces launched short-range ballistic missiles of their own less from the peninsula into the Sea of Japan in a show of force.More news: Auto lender Ally Financial reports higher-than-expected earnings
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One reason why Pyongyang did not carry out a missile test on Thursday could be that it rained, according to local experts, noting that missile parts are highly sensitive to humidity.
North Korea decided not to go ahead with a missile launch to mark the 64th anniversary of an armistice that brought a ceasefire of the Korean War in July 27, 1953, ... but there are concerns escalating tensions with Washington may play down Seoul's efforts to improve inter-Korean relations.
North Korea has made no secret of its plans to develop a nuclear-tipped missile capable of striking the United States and has ignored worldwide calls to halt its weapons programs.
The South Korean government formally asked the North on July 17 to discuss ways to cease all "acts of hostility" near the border in bilateral talks, specifically suggesting July 21 as a possible date.
ROK is the acronym of South Korea's official name, the Republic of Korea. The last time a family reunion was held was in October 2015.