North Korea has slammed the US-led diplomatic boycott of February's Beijing Winter Games, state media reported Friday, describing it as an "insult" to the Olympic spirit.

Washington and some of its allies have announced a diplomatic boycott, citing China's human rights record, in a decision that incensed Beijing.

China is the main ally and economic benefactor of impoverished North Korea, which is suspended from competing in the Chinese capital.

North Korean Olympic and sports bodies criticised the boycott in a letter to their Chinese counterparts, the official Korean Central News Agency (KCNA) said.

"The US and its vassal forces are getting ever more undisguised in their moves against China aimed at preventing the successful opening of the Olympics," the letter said, according to KCNA.

Their moves are "an insult to the spirit of the international Olympic Charter".

North Korea stayed away from the pandemic-delayed Tokyo Games last year, refusing to send its athletes over Covid-19 concerns.

The International Olympic Committee punished the no-show with a suspension that meant it could not participate in the Beijing Winter Games.

Despite being barred, North Korea depicted its absence as the result of the pandemic and "hostile forces".

The KCNA report did not identify the "hostile forces" and there was no mention of the IOC suspension.

China's foreign ministry said Friday the North Korean absence was due to "special reasons" and expressed its "full understanding".

The IOC said in September, when it announced the punishment, that Pyongyang had rejected all coronavirus safety proposals -- including the provision of vaccines -- ahead of the Tokyo Olympics.

North Korea imposed a rigid Covid blockade -- among the strictest in the world -- in the early days of the pandemic and has not since eased it.

The country's absence from Beijing marks a huge shift from the last Winter Games, in South Korea, where it had the attention of the world.

Leader Kim Jong Un's sister attended the 2018 Pyeongchang Games as his envoy in a blaze of publicity and South Korean President Moon Jae-in seized the opportunity to broker talks between Pyongyang and Washington.