Why are some Christians being discriminated against in the United States? For Republican presidential nominee Donald Trump, it's the media's fault.

In a speech before social conservatives in Washington on Friday, the controversial businessman criticised American media organisations for their supposed mockery of Christians while promising to champion religious liberty if he wins the U.S. presidency this November.

"Our media culture often mocks and demeans people of faith," Trump said, as quoted by The Hill.

"All the time, I hear from concerned parents how much harder it is for Christian families to raise their children in today's media environment. Your values of love, charity and faith built this nation. So how can it be that our media treats people of faith so poorly?" he added.

Trump also attributed the "poor" media treatment of Christians to his rival, Democratic Party presidential nominee Hillary Clinton.

"One of the reasons is that our politicians have abandoned you to a large extent, and Hillary Clinton you can forget about her," the Republican presidential bet said.

As part of his platform, Trump also promised to allow churches and church-based organisations to endorse political candidates by repealing the Johnson Amendment.

"The first thing we have to do is give our churches their voice back," Trump said. "The Johnson Amendment has blocked our pastors from speaking their minds from their own pulpit. If they want to talk about Christianity, if they want to preach or talk about politics, they're unable to do so, they take a tremendous risk that they'll lose their tax-exempt status."

The Republican candidate also spoke about the Islamic State and its persecution of Christians. He vowed to prioritise "hunting down and exterminating" members of the terror group.

"ISIS is carrying out a genocide against Christians in the Middle East and we cannot let this evil continue. ISIS must be destroyed," Trump said.