Former US President Donald Trump has once again pressured NATO allies to shoulder more defense costs, saying his country would not defend them otherwise.

During an election campaign rally in South Carolina on Wednesday, Trump said that NATO nations "weren't paying their bills." He added, "If they're not going to pay, we're not going to protect."

The following day, Trump insisted again that the United States is paying a large amount of defense expenditures and said NATO allies are laughing at the stupidity of his country.

Trump revealed on Saturday that while he was in office he said that the US might not protect its NATO allies from a potential Russian attack, unless they commit to their fair share of defense expenditures.

His remarks on Saturday sparked backlash from NATO nations. US President Joe Biden also said Trump had sent a dangerous signal to the world.