The US and South Korea agreed on Sunday to replace two major war games that take place every spring -- the Key Resolve and Foal Eagle drills -- with a shorter "Dong Maeng" or "Alliance" exercise which kicked off this week.

The move was designed to further ease tensions with the North following the dramatic detente since early 2018.

There are close to 30,000 US troops stationed in South Korea, and their annual drills with tens of thousands of South Korean soldiers have always infuriated the North -- with Pyongyang condemning the manoeuvres as provocative rehearsals for invasion.

However, following the first meeting between US President Donald Trump and North Korean leader Kim Jong Un in Singapore last year, Trump raised eyebrows at a press conference when he said Washington would suspend the "very provocative" US joint military exercises with South Korea.

The two leaders also signed a vaguely-worded pledge on denuclearisation of the Korean peninsula.

But the North's official KCNA news agency has now warned that the ongoing nine-day drills were an "open challenge" against efforts for peace and stability.

"The ill-boding moves of the South Korean military authorities and the US are a wanton violation of the DPRK-US joint statement and the north-south declarations in which the removal of hostility and tensions were committed to," it said, using the acronym for the North's official name.

The flare-up comes just days after Trump and Kim Jong Un held a second summit, this time in Vietnam, but the talks broke up early with no progress toward Washington's goal of getting the isolated country to give up its nuclear weapons.

Following the stalemate, researchers said this week that Pyongyang was rebuilding the Sohae long-range rocket site after Kim had agreed last year to shut it as part of confidence-building measures.

Trump said he would be "very, very disappointed" if the reports proved true.