US President Donald Trump on Friday told investors who he said were investing money in the United States that his policies would never change, as global stock markets tumbled for a second day following his announced tariff plan and China's retaliatory measures.

"To the many investors coming into the United States and investing massive amounts of money, my policies will never change. This is a great time to get rich, richer than ever before!!!" he wrote in all caps in a post on his social media platform.