The influential sister of North Korean leader Kim Jong Un called Seoul authorities "perfidious" Tuesday over the South's joint military exercises with the United States and demanded Washington withdraw its forces from the peninsula.

Kim Yo Jong's latest remarks come despite a surprise thaw on the Korean peninsula, prompted by a series of personal letters between her brother and South Korean President Moon Jae-in.

The two sides last month restored cross-border communications that were severed more than a year ago, announcing their leaders had agreed to work on improving ties.

But Kim Yo Jong -- a key adviser to her brother -- condemned South Korea for holding "dangerous" joint military drills with the United States this month, which the North has long considered rehearsals for invasion.

She expressed her "deep regret at the perfidious behaviour of the south Korean authorities" in a statement released by Pyongyang's official KCNA news agency.

Kim Yo Jong
Kim Yo Jong is a key adviser to her brother Photo: POOL / JORGE SILVA

In an unusually explicit comment from a North Korean official -- Pyongyang usually restricts itself to ambiguous calls for the United States to abandon its "hostile policy" -- she demanded Washington pull out its forces from the peninsula.

Seoul and Washington are treaty allies, with around 28,500 American troops stationed in South Korea to defend it against its nuclear-armed neighbour.

"For peace to settle on the peninsula, it is imperative for the US to withdraw its aggression troops and war hardware deployed in south Korea," Kim said.

Her comments came as the US and South Korean militaries began their preliminary training Tuesday in the run-up to next week's yearly summertime exercise.

They have already scaled back their annual joint military exercises significantly to facilitate nuclear talks with Pyongyang.

US-Korea military drills
US Army M109A6 Paladin self-propelled howitzers take part in a live firing drill in Pocheon, south of the Demilitarized Zone that divides the two Koreas, in 2012 Photo: POOL / Kim Hong-Ji

Asked about Kim Yo Jong's remarks, US State Department spokesman Ned Price said that the exercises were part of the "ironclad" alliance with South Korea.

"Let me reiterate that the joint military exercises are purely defensive in nature," Price told reporters.

"As we have long maintained, the United States harbours no hostile intent towards the DPRK," he said, using the North's official name of the Democratic People's Republic of Korea.

"We support inter-Korean dialogue, we support inter-Korean engagement and we'll continue to work with our ROK partners towards that end," he said, referring to South Korea.

Seoul's defence ministry said the North did not answer the daily calls made between the two Koreas on their military hotline on Tuesday afternoon, just two weeks after the communication link was restored.

The South's dovish Moon is credited with brokering the first-ever summit between North Korea and a sitting US president, in Singapore in June 2018.

But Pyongyang largely cut off contact with Seoul following the collapse of a second summit between Kim and then US president Donald Trump in Hanoi that left nuclear talks at a standstill.

Cheong Seong-chang, director of the Center for North Korean Studies at the Sejong Institute in Seoul, pointed out the North had in the past raised tensions during the South's joint military drills with the United States.

"And they would suddenly switch to a policy of appeasement whenever it was deemed necessary, when the drills were over," he said.

Kim Yo Jong had accused Washington of hypocrisy, he added, but she had "no right to say that as North Korea refuses to even be contacted by the US".

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