The warning comes amid a surprise thaw on the Korean peninsula, prompted by a series of personal letters between Kim and South Korean President Moon Jae-in.

The two sides on Tuesday 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 the North Korean leader -- warned the mood could shift if the South holds joint military drills with the United States later this month.

"I view this as an undesirable prelude which seriously undermines the will of the top leaders," Kim Yo Jong said, adding the drill "further beclouds the way ahead of the north-south relations".

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, which condemns such drills as preparations for an invasion.

The 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.