U.S. President Donald Trump has threatened Yemen's Houthi rebels that they'll be "completely annihilated" as American airstrikes pounded locations under their control, while further pressuring the group's main benefactor Iran.

"Any act of aggression will have severe consequences, for which the U.S. will bear full responsibility," Iranian Permanent Representative to the United Nations Amir Saeid Iravani wrote in a letter to U.