Kim Jong-un's powerful sister has made a rare public statement to warn new US President Joe Biden not to "cause a stink".
Through North Korea's state media, Kim Yo-jong criticised the US and South Korea for holding joint military drills across the border on the Korean peninsula.
She threatened to end a military peace agreement with the South and disband a team tasked with engaging with officials in Seoul.
It comes after the Biden administration said it had been trying for weeks to begin a discussion with Pyongyang as it sets out its policy on North Korea, which has refused to acknowledge the Democrat.
After famously trading insults, Kim Jong-un had a friendly relationship with disgraced former president Donald Trump, with the pair setting foot in North Korea during one of their three meetings.
It is unlikely the relationship between Kim and Biden will be as warm - the president called the dictator a "thug" during last year's election campaign.
In North Korea's first public message to Washington since Biden was sworn in on January 20, Kim Yo-jong, one of her brother's closest advisers, was the one who issued the secretive state's latest threat to America.
Despite her rising influence, there has been speculation about her status and whether there was a power struggle in Pyongyang.
A day before top US officials arrive in Seoul for talks, she warned Washington that the smell of cordite, wafting over the border from the joint military drills, would not help bring peace.
Kim Yo-jong, thought to be aged 33, said: "We take this opportunity to warn the new US administration trying hard to give off powder smell in our land.
"If it wants to sleep in peace for coming four years, it had better refrain from causing a stink at its first step."
She repeated the North's opposition to the military exercises, which it sees as preparation for an invasion.
In comments translated by Yonhap News Agency, she said: "South Korea has again chosen 'March of war' and 'March of crisis' instead of 'warm March' in front of all Korean people.
"Whatever the South will do by following its master, it will be not easy that the warm spring days of three years that it strongly wants will return."
She added: "War drills and hostility can never go with dialogue and cooperation."
Referring to a deal signed to ease tensions with Seoul, she said: "We will keep an eye on the attitude and behavior of South Korea going forward and should it become more provocative, we could take special measures such as boldly scrap the inter-Korean military agreement."
The latest drill by the American and South Koreans involved computer simulations due to the risk posed by coronavirus and continuing efforts to have a diologue with the North.
US Secretary of State Antony Blinken, who is to visit Seoul with Defense Secretary Lloyd Austin, told reporters in Tokyo that he was aware of Kim Yo-jong's comments.
He said he was more interested in hearing what America's allies and partners think about North Korea.
The Biden administration is crafting its policy on North Korea and is expected to reveal more in coming weeks.
Blinken said Washington is considering whether additional pressure on North Korea could be effective.
South Korea defended the military drills.
An official told reporters in Seoul: "The government will endeavor till the end to make the joint military drills be carried out in a way that the peace process (on the Korean Peninsula) could be supported.
"We will keep making efforts for talks and cooperation."
The North is already subject to strict sanctions, most of which concern its nuclear weapons programme.
Ramon Pacheco Pardo, a Korea expert at King's College London, said the North will likely be at the top of the US officials' agenda when they meet their South Korean counterparts.
He told Reuters: "Until now, the discussion was focusing on The Quad, dealing with China and the North Korea policy review.
"Kim's statement will be central to discussions."
The White House said Pyongyang has rejected its attempts to begin a dialogue.
After a third summit between Trump and Kim Jong-un, North Korea ended talks and said it would not engage again until the US dropped its "hostile" policies.
In January, the name of Kim's sister was missing from a new list of the ruling Workers’ Party’s politburo, raising questions about her status after several years of increasing influence.
It came as Kim cemented his power at a congress with his election as party general secretary, taking the title from his late father, Kim Jong Il.
He has wielded almost absolute power in North Korea’s dynastic system since taking over following the death of his father in 2011.
