Myanmar and China should worry about the consequences of persecuting minorities, the United States envoy for religious freedom, Mr Sam Brownback, has warned in an interview with The Straits Times.

Close to a million of Myanmar's minority Rohingya are now in refugee camps in Bangladesh, following a pogrom in 2017 by Myanmar's military.

Mr Brownback, the US Ambassador-at-Large for International Religious Freedom, said: "You're certainly hearing a lot of discussion (in Congress) about increased actions by the United States, and if this gets going... you don't know where those things will end up.

He said it really behoves Myanmar "to act responsibly and act now… because as long as these people are forced out of their country and are stateless, you will see international reaction and I think it will grow over time".

"This has been going on for decades and it's not just the Rohingya," he said. "You've got the Kachin and the Karen that are mostly Christian, being forced out as well."

Stateless people in the region are at grave risk of human trafficking, he added in the interview on Monday at his State Department office.

The former governor of Kansas left the state for the US capital last year after a controversial tenure in which his tax policies backfired and his conservatism on abortion and gay rights drew flak.

He was speaking on the heels of the release of the State Department's 2018 International Report on Religious Freedom, which criticises Myanmar, China and Saudi Arabia, among others.

The report cites China's detention of over a million ethnic Uighur Muslims in Xinjiang province, and reported intolerance of Christians and Tibetan Buddhists.

Mr Brownback said "80 per cent of the world community is focused on a faith of some type".

China, at a time when it is trying to project itself as a world leader, has "a million people under detention, is knocking down churches, is destroying Buddhist monasteries", he said. "China has declared a war on faith," he added.

The country should address issues now, before they "fully erupt in ways you do not know", he warned.

"I think we are at the front end of what consequences come out of it, which are by and large unpredictable. I would argue they are going to create more terrorism. Our data says China is going to create more problems than they are going to cure."

China's Foreign Ministry on Monday slammed the religious freedom report, urging the US to stop using religion to interfere in China's domestic affairs.

"The so-called report and the recent remarks by US Secretary of State Mike Pompeo, which are related to China, neglect the truth and are full of ideological bias," spokesman Geng Shuang said at a press conference. "They slander China's religious policies and the policies in Xinjiang and openly interfere with China's internal affairs."

China maintains that radicalism is a problem in the Uighur population, but claims the large detention centres in Xinjiang are re-education camps and the Uighurs there can come and go as they please.

Myanmar says the Rohingya are historically migrants from Bangladesh, and not entitled to citizenship in Myanmar. The government does not recognise the term "Rohingya", insisting it is an invented identity. It refers to the Rohingya as Bengalis.

The US has sanctioned some Myanmar generals over the pogrom of 2017. There is also legislation pending in Congress that would target Chinese involved in the mass detentions of Uighur as well.

"The Rohingya should be allowed to return freely and safely," Mr Brownback said. "I think the world community needs to continue to keep pressure on Burma (Myanmar) and its allies to force them to do what's right."

It is not clear if the sanctions on Myanmar are having any effect on the position of the Myanmar government, he said. But sanctions are a valuable way for the international community to express its displeasure.

The report was evenhanded, Mr Brownback insisted - and the US would continue to promote international norms even if the problems identified were long term and sometimes seemingly intractable.

"We can't just back away and say, well that's the way it is," he said.