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Any North Korean capability to field submarine-launched ballistic missiles in open waters would be particularly worrying for the United States and its allies, since such missiles are hard to detect before launching. In August 2016, North Korea successfully tested such a missile from near its submarine base in Sinpo, sending it 310 miles toward Japan in a launch that came after several failed tests.

The military maneuvers come a week after the United States and South Korea began holding joint drills that included flyovers of advanced stealth fighters and B1-B Lancer bombers over the Korean Peninsula, exercises that led North Korea to accuse the United States of pushing the region “to the brink of nuclear war.”

To step up pressure, South Korea on Monday imposed a new round of sanctions on 20 North Korean groups and 12 individuals. The sanctions, an effort to curtail North Korea’s missile and nuclear weapons programs, were imposed in retaliation for the North’s launching of a missile in late November that experts said was capable of hitting much of the continental United States.

Separately, Nikki R. Haley, the U.S. envoy to the United Nations, said over the weekend that the United States would send a full delegation of athletes to the Winter Olympics in South Korea in February. Her remarks on “Fox News Sunday” came days after she sowed doubt about participation in the games by saying it was an “open question” whether American athletes would participate, given the tensions on the Korean Peninsula.