U.S. Secretary of State Mike Pompeo attacked China's record on religious freedom on Wednesday, during a visit to Rome that has been overshadowed by his criticism of the Vatican for pursuing closer ties with Beijing.

"Nowhere is religious freedom under assault more than in China," Pompeo told a symposium hosted by the U.S. Embassy to the Holy See, saying the Chinese Communist Party was looking to "to snuff out the lamp of freedom ... on a horrifying scale".

Pompeo, an evangelical Christian who has portrayed himself as a champion of religious rights, denounced Beijing's treatment of its Uighur Muslim minority, and said all religious groups faced repression.

"The Chinese Communist Party has battered every religious community in China, Protestant house churches, Tibetan Buddhists, Falun Gong devotees and more. Nor of course have Catholics been spared this wave of repression," he said.

China has consistently defended its human rights record and has denied any mistreatment of Uighurs.

Vatican officials expressed surprise last week when, ahead of his planned visit to Rome, Pompeo published an essay in a conservative Catholic magazine that sharply criticised the Holy See for plans to renew a two-year-old agreement with Beijing.

Pompeo said the deal, which gives the pope some say over the appointment of Chinese bishops, endangers the Vatican's moral authority. Vatican officials say that while the arrangement is not perfect, it is an improvement after decades during which Chinese Catholics who recognise the pope were forced underground.

The administration of President Donald Trump has made its hard line on China a central issue in the campaign for the Nov. 3 presidential election. Trump is also strongly aligned with conservative Protestant and Catholic movements, including some that are openly critical of Pope Francis.

Pompeo is due to hold talks on Thursday with Cardinal Pietro Parolin, the Vatican's top diplomat. Pope Francis saw Pompeo when he came to Italy last year but no such meeting is scheduled this time around.