A US rights monitor sounded the alarm Monday over athletes' safety at China's upcoming Winter Olympics, after the host threatened "punishment" for anti-Beijing comments.

The Congressional-Executive Commission on China -- a group of Washington lawmakers and White House officials -- asked US Olympics authorities for an "urgent effort" to protect their sports stars' free speech rights at the February 4-20 event.

The commission spoke out after Yang Shu, a senior official in the Beijing Games organizing committee, told an online briefing on January 18 that "any behavior or speech that is against the Olympic spirit, especially against the Chinese laws and regulations, are also subject to certain punishment."

"Given Yang Shu's statement and the Chinese government's documented behavior, we urge the USOPC to be vigilant and prepared to defend any Olympians who speak out," the commission said in a letter to the US Olympic and Paralympic Committee seen by AFP.

The United States, Australia, Canada and Britain announced in December that they would not be sending official representatives to Beijing, citing the alleged genocide and crimes against humanity against the Uyghur population, a minority Muslim group in the Xinjiang region.

The countries said their athletes would still participate, but China nevertheless responded angrily, warning Western nations that they would "pay the price" for the protest.

Advocacy groups have backed the boycott, with Human Rights Watch (HRW) China director Sophie Richardson calling it a "crucial step toward challenging the Chinese government's crimes against humanity targeting Uyghurs and other Turkic communities."

'Gold medal for censorship'

Campaigners say that at least one million Uyghurs and other Turkic-speaking, mostly Muslim minorities have been incarcerated in "re-education camps" in Xinjiang, where China is also accused of forcibly sterilizing women and imposing forced labor.

Beijing has defended the camps as vocational training centers aimed at reducing the appeal of Islamic extremism.

The commission also voiced concerns over Chinese tennis star Peng Shuai, who made sexual assault allegations against a top Communist Party official.

Peng was not heard from for nearly three weeks, and there remain concerns about how free she really is.

New York-based HRW awarded China a "gold medal for censorship" last week, in a statement accusing the Asian giant of trying to "sportswash" its "abysmal human rights record."

"Athletes participating in the games will be surveilled, and their rights to free speech and protest severely curtailed," said Director of Global Initiatives Minky Worden.

Asked whether President Joe Biden supports US athletes making political statements at the Olympics, White House Press Secretary Jen Psaki told reporters the administration had "conveyed support for freedom of speech of individuals."