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 -- said the South was "treacherous" for holding "dangerous" joint military drills with Washington this month, which the North has long considered rehearsals for invasion.

"I take this opportunity to express my strong regret for the treacherous treatment of the South Korean authorities," she said in a statement released by Pyongyang's official KCNA news agency.

Kim's statement 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 "surely face greater security threats", 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.