Iran's foreign minister, Mohammad Javad Zarif, accused President Trump of planning to commit a war crime against the Islamic Republic of Iran, echoing criticism leveled earlier by Rep. Alexandria Ocasio-Cortez (D-NY).
Zarif was responding to Trump's threat to target 52 identified Iranian sites, some with cultural significance to Iran, should the Islamic Republic retaliate against the United States for the recent killing of Qasem Soleimani. Zarif described the killing of Soleimani as a breach in international law, claiming future attacks against culturally significant sites in Iran would constitute a war crime.
Those masquerading as diplomats and those who shamelessly sat to identify Iranian cultural & civilian targets should not even bother to open a law dictionary.— Javad Zarif (@JZarif) January 5, 2020
Jus cogens refers to peremptory norms of international law, i.e. international red lines. That is, a big(ly) "no no".