Kim Jong Un's mysterious sister, Kim Yo Jong, issued a rare public threat to the US on her brother's behalf, hinting that she might be gaining influence

• Kim Jong Un's sister threatened the US against intercepting North Korean missile tests on Tuesday.
• Kim Yo Jong comments come as the US and South Korea ramp up military and combat drills.
• Ms. Kim's presence in her brother's government is mysterious, but she may be gaining influence.

Kim Jong Un's sister Kim Yo Jong threatened the US on Tuesday against intercepting any North Korean missile tests over the Pacific Ocean, calling any interference a "declaration of war."

It's a rare public warning from Kim Yo Jong — and a sign she could be gaining more influence in her brother's government. 

According to a press release on the Korean Central News Agency, Kim said the Pacific "does not belong to the dominium of the US or Japan" and reaffirmed North Korea's right to test weapons in the area.

Kim specifically referenced the country's plans to test intercontinental ballistic missiles. 

"It will be regarded as a clear declaration of war against the DPRK, in case such military response as interception takes place against our tests of strategic weapons," Kim said.

Her involvement in her brother's government has been a mystery, but she's been more public recently.

The Wall Street Journal reported that Kim Yo Jong has long been seen as Kim Jong Un's unofficial second-in-command and a potential heir to North Korea's leadership if her brother should become incapacitated or die suddenly. 

One of her most public appearances came in 2018 when Kim became the first member of North Korea's ruling family to visit South Korea for the Pyeongchang's Winter Olympics, the Journal said. 

Kim Jong Un has appointed a variety of women to prominent roles in his regime. Experts believe the increasing prominence of his daughter Kim Ju Ae, who is believed to be 10 or 11 years old, may signal that he is preparing her as an heir. 

Meanwhile, tensions have ramped up in the area as the US and South Korean militaries conduct training exercises nearby. The Associated Press reported that one drill on Monday involved the US and South Korea flying a B-52 bomber over the Korean Peninsula. 

The allies also simulated a North Korean nuclear crisis for a tabletop exercise in February. In the coming weeks, their militaries have a variety of drills planned including their largest field exercises ever.