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 the South for holding "dangerous" joint military drills with Washington 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's 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.

By carrying out the drills, Seoul and Washington would face a "more serious security threat", she said, and the North would strengthen its defence and pre-emptive strike capabilities.

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

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

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 the North 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 previously raised tensions during the South's joint military drills with the US.

"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".