Nikki Haley has warned against taking the side of “a thug who kills his opponents” after comments made by Donald Trump.
She was speaking after Donald Trump said he would encourage Russia to attack Nato members who “failed to pay”.
Ms Haley and Mr Trump are hoping to be the Republican Party’s candidate in the 2024 US presidential election.
Nato’s chief has suggested the former US president’s comments put US and European troops at greater risk.