Is it against the law in America to threaten to kill the leader of a foreign country?

The legal jurisdiction of the United States is generally the United States.
Would it be illegal to threaten a foreign leader? Such as Bolsanaro in Brazil? or Prince Mohammed bin Salman in Saudi Arabia?
Of course it would be illegal to threaten them, if they were visiting the USA, but I’m talking about threatening them while the threat-maker is in America and the threat-ee is in their home country.