Tehran said Monday it was keeping communication channels with Washington open as U.S. President Donald Trump considered how to respond to Iran¡¯s deadly crackdown on nationwide protests ¡ª one of the gravest tests of clerical rule since the Islamic Revolution in 1979.

Adding to threats of military action, Trump late on Monday announced that any country doing business with Iran, a major oil producer, will face a new tariff of 25% on its exports to the U.S.

¡°This Order is final and conclusive,¡± Trump wrote in a social media post, without providing further detail about the legal authority he would use to impose the tariffs, or whether they would be aimed at all of Iran¡¯s trading partners. The White House did not respond to a request for comment.