President Trump said Iran is not living up to the "spirit" of its agreement forged with the Obama administration.
Speaking in Washington with Italian Prime Minister Paolo Gentiloni, Trump said the agreement should not have been signed.
He said the Iranian government is doing a "tremendous disservice" to the JCPOA agreement, often dubbed the "Iran nuclear deal."
Trump also took aim at North Korea, calling its dictator, Kim Jong-un, a "menace."
He also praised China for its recent actions that appear to show Beijing taking a tougher stance against the country.