The US ambassador for religious freedom on Monday again urged China to give its people religious freedom, a move he said could help gain trust from self-ruled Taiwan, an island China considers its own.

Sam Brownback on Friday said in a speech in Hong Kong that China was waging 'war with faith' and that it needed to respect the 'sacred right' of people to worship, especially Muslims locked up in internment camps in Xinjiang.

On Monday, he addressed the issue of Taiwan which China considers a wayward province and has pledged to unite, by force if necessary.

'If they want to build some confidence in Taiwan, they should give religious freedom to their own people,' he told Reuters. 'If they would give religious freedom to their own people, that would be noticeable.'

Chinese President Xi Jinping sought to reassure people in Taiwan in January that religious and legal freedom on the island would be respected under a peaceful 'reunification'.

Taiwan President Tsai Ing-wen has in return rejected Xi's call and instead urged China to embrace democracy.

Brownback's visit to Taipei was viewed by some in Taiwan as a sign of support from the Trump administration amid growing friction between Taipei and Beijing.

The United States switched diplomatic recognition from Taiwan to China in 1979, acknowledging that China takes the position that there is 'one China' and Taiwan is part of it.

But the United States is also Taiwan's biggest ally and arms supplier and is duty-bound by legislation to help the island defend itself.

In Beijing, Foreign Ministry spokesman Lu Kang said China opposed any kind of official contact between the United States and Taiwan.

China was strongly opposed to the envoy's 'wrong comments' about religious freedom and had lodged 'stern representations' with the United States, Lu said, adding that China protected religious freedom.

China opposes the United States using the issue of religion to interfere in its internal affairs and urged it to stop, he said.

China runs vocational training centres in Xinjiang that meant to carry out de-radicalisation and prevent terror, Lu said.

'They are absolutely not, as the United States exaggerates, so-called re-education camps.'