US President Donald Trump has warned that countries aligning with the BRICS economic bloc will face additional 10 per cent tariffs on their goods, as he seeks to protect the dollar's global dominance.

Speaking at a White House event on Friday, Trump accused the group of trying to "take over" the US dollar and said he would not allow America's currency to "slide".

"We have this little group called BRICS, which is fading out fast," the president said during a ceremony where he signed cryptocurrency legislation into law. "BRICS, they wanted to try and take over the dollar, the dominance of the dollar and the standard of the dollar."