Pentagon chief Lloyd Austin and Secretary of State Antony Blinken arrived in Japan on Monday on their first overseas trip, aimed at rallying military alliances as a bulwark against China and cementing a united front against the nuclear-armed North.

North Korean leader Kim Jong Un's influential sister warned the United States against actions that could make it "lose sleep", state media reported Tuesday, as top Biden administration officials began a visit to key allies Tokyo and Seoul.

Pentagon chief Lloyd Austin and Secretary of State Antony Blinken arrived in Japan on Monday on their first overseas trip, aimed at rallying military alliances as a bulwark against China and cementing a united front against the nuclear-armed North.

The statement by Kim Yo Jong, a key adviser to her brother, was Pyongyang's first explicit reference to the new president in Washington, more than four months after Joe Biden was elected to replace Donald Trump -- although it still did not mention the Democrat by name.

The United States and South Korea began joint military exercises last week and Pyongyang's official Rodong Sinmun newspaper carried a statement from her offering "a word of advice to the new administration of the United States that is struggling to spread the smell of gunpowder on our land".

"If you wish to sleep well for the next four years, it would be better not to create work from the start that will make you lose sleep," she said.

Trump's unorthodox approach to foreign policy saw him trade insults and threats of war with Kim Jong Un before an extraordinary diplomatic bromance that saw a series of headline-grabbing meetings.

But ultimately no progress was made towards Washington's declared aim of denuclearising North Korea, which is under multiple international sanctions for its banned weapons programmes.

It has isolated itself further, imposing a strict border closure to protect itself against the coronavirus pandemic that first emerged in neighbouring China.

Shortly before Biden's January inauguration, leader Kim decried the US as his country's "foremost principal enemy" and Pyongyang unveiled a new submarine-launched ballistic missile at a military parade.

- 'March of war' -

The talks process was brokered by South Korea's President Moon Jae-in but relations between Seoul and Pyongyang have been in deep freeze since Kim and Trump's summit in Hanoi collapsed in February 2019.

Kim Yo Jong is a trusted adviser to her brother and was a key voice when inter-Korean tensions mounted last year, culminating in the North blowing up a liaison office on its side of the border.

Shin Beom-chul, a researcher at the Korea Research Institute for National Strategy, pointed out that her announcements have previously represented incremental steps by Pyongyang.

"North Korea has judged that the US will not offer enough concessions and so has released this statement ahead of Blinken and Austin's visit to Seoul," he told AFP.

There was a "high possibility" of a military provocation by the North during or immediately after the Americans' trip, he added.

Seoul and Washington are treaty allies, with the United States stationing around 28,500 troops in South Korea to defend it against its neighbour, and they began computer-simulated joint military exercises last week.

The North always condemns such drills as preparations for invasion, and in her statement, Kim Yo Jong said: "The South Korean government yet again chose the 'March of War', the 'March of Crisis' rather than a 'warm March' before all the people."

"It will not be easy for the warm spring days of three years ago to come back if the South Korean government follows whatever instructions of its master," she added, threatening to scrap a North-South military agreement if Seoul acts "more provocatively".

- New York channel -

Austin and Blinken arrived in Tokyo on Monday and will be consulting with both it and Seoul on the new administration's review of US policy towards Pyongyang, but China will be the main focus of their discussions.

Beijing presents multiple challenges to Washington on trade and diplomatic fronts, and is also the North's key diplomatic backer and main provider of business and aid.

Washington has attempted to reach out to Pyongyang "through several channels starting in mid-February, including in New York", state department deputy spokesperson Jalina Porter told reporters on Monday.

"To date, we've not received any response from Pyongyang," she added.

The "New York channel" is a reference to the North's mission to the United Nations, as Pyongyang and Washington do not maintain diplomatic relations.

Austin and Blinken are due to arrive in South Korea on Wednesday before the defence secretary heads to India while the diplomat returns to the United States for talks with Chinese officials.