U.S. President Donald Trump on Tuesday expressed confidence in Treasury Secretary Steven Mnuchin amid worries over a weakening economy and a stock market slump, but repeated his criticism of the U.S. Federal Reserve, saying it has raised interest rates too quickly.

Speaking to reporters in the Oval Office after a Christmas video conference with U.S. troops deployed abroad, Trump also said U.S. companies were "the greatest in the world" and presented a "tremendous" buying opportunity.

Asked if he has confidence in Mnuchin, Trump said: "Yes, I do. Very talented guy. Very smart person," he said. His comments came after Mnuchin on Monday held a conference call with U.S. regulators to discuss plunging U.S. stock markets.