A menacing warning North Korea is threatening "military measures" to punish South Korea for its propaganda campaign isn't alarming only because of its blatant threat of military force. It's startling because it was delivered by Kim Yo-jong, the younger sister of dictator Kim Jong-un.

North Korea watchers say what we might be seeing is a gradual devolution of power to Yo-jong as Jong-un's successor. Yo-Jong is currently first vice director of the United Front Department of the Workers' Party of Korea (WPK). She's the youngest child of the late dictator, Kim Jong-il. She wed Choe Song in 2014 and gave birth to a child in March 2015.

Her public visibility was vastly increased in April when her brother disappeared from public view for more than two weeks. Her brother has since appeared in public, albeit only a few times over the past months. Jong-un's continuing invisibility is again fueling speculation of a severe health problem.

John Park, director of the Korea Project at the Harvard Kennedy School, surmises what we're seeing with Yo-Jong's high-profile ascent is a reflection of the partnership she has with her brother.

"It looks like there is this kind of power partnership between the two that became more visible," said Park.

He noted the fact Yo-jong is issuing provocative statements on her brother's behalf confirms growing speculation she might soon succeed him despite opposition from some quarters wary of having a woman in charge of North Korea for the first time.

Park said since we're seeing a lot of big statements coming out of Yo-jong, this indicates her role isn't just ceremonial and she's been chronically underestimated.

What's raised the world's eyebrows are statements made by Yo-jong warning of military attacks against South Korea for a propaganda campaign launched by South Koreans and North Korean defectors urging North Koreans to defect. The campaign, which has been going on for decades, floats balloons carrying propaganda and rice northward. It also uses bottles to deliver messages to defect.

On Saturday, Yo-jong threatened retaliatory military measures against South Korea. She said by exercising her power "authorized by the Supreme Leader, our Party and the state, I gave an instruction to the ... department in charge of the affairs with (the) enemy to decisively carry out the next action."

She also threatened to destroy joint liaison office at the Kaesong industrial zone straddling the demilitarized zone (DMZ). She also invoked the authority of her brother to order the Korean People's Army "to prepare for possible military action against what we believe to be the inter-Korean liaison office."

Park believes this sudden outburst of bellicosity from North Korea reflects its immense frustration two years of diplomacy with president Donald Trump has not eased international sanctions on North Korea. The second anniversary of Jong-Un and Trump's first meeting in Singapore took place June 12, and Yo-jong's threat of war was made the next day.

North Korean watchers say Yo-jong's threat should be seen more as expressions of anger and frustration at Trump.