U.S. President Donald Trump is warning that Washington will walk away from a nuclear deal it agreed to with Iran if it deems that the U.N. agency monitoring the agreement is not tough enough in monitoring it.

In a message Monday to a meeting of the International Atomic Energy Agency read by U.S. Energy Secretary Rick Perry, Trump says "we will not accept a weakly enforced or inadequately monitored deal."

But Iran says the greatest threat to the deal is "the American administration's overly hostile attitude." Alluding to U.S. assertions that the deal allows the IAEA to inspect Tehran's military sites, Iranian nuclear chief Ali Akbar Salehi is urging the agency to "resist such unacceptable demands."