Donald Trump was being "light-hearted" when he told Mexican President Enrique Pena Nieto that he might send US troops to deal with "bad hombres down there" if the Mexican military does not, a White House source has said.
The official says the remark was meant to be a reference to co-operation between the countries in fighting drug cartels.
The Associated Press first reported on Wednesday that US President Mr Trump made the comment in an hour-long conversation with Mr Pena Nieto on January 27.
The official added: "Those comments, while light-hearted, were part of a discussion about how the United States and Mexico could work collaboratively to combat drug cartels and other criminal elements, and make the border more secure."
The official described the conversation as "pleasant and constructive".