"As we've been starkly reminded in recent days, North Korea continues to present an extraordinary threat to the region and which demands our continued vigilance," David Helvey, the acting assistant secretary of defense for Indo-Pacific security affairs, told reporters.
Helvey said that it was difficult to tell what North Korea would do in the coming days and weeks.
North Korea on Tuesday blew up a joint liaison office set up in a border town in 2018 to foster better ties with South Korea after threatening action if defectors continued with a campaign of sending propaganda leaflets into the reclusive North.
Destruction of the building, closed since January due to coronavirus fears, represented a major setback to South Korean President Moon Jae-in's efforts to coax North Korea into cooperation. It also appeared to be a further blow to US President Donald Trump's hopes of persuading North Korea to give up its nuclear weapons and open up to the world.
North Korean leader Kim Jong Un's sister, Kim Yo Jong, on Wednesday criticized Moon for failing to implement a 2018 peace accord, saying Moon "put his neck into the noose of pro-US flunkeyism."North Korea last week warned Washington to refrain from commenting on inter-Korean affairs if it wanted the Nov. 3 US presidential election to go smoothly, raising concerns it could be contemplating a return to nuclear and long-range missile testing.