Iran’s Supreme Leader Ayatollah Ali Khamenei vowed “severe retaliation” awaits the killers of one of the country’s top commanders Qassem Soleimani in Iraq.

Soleimani, who led proxy militias that extended the Iran’s reach across the Middle East, was killed in a strike authorized by U.S. President Donald Trump. The attack occurred near Baghdad international airport and details remain unclear, and there are reports that another Iraqi militia leader, Abu Mahdi al-Muhandis, was also killed.

Khamenei said in statement that a “severe retaliation awaits murderers who have the blood of Soleimani and that of other martyrs on their wicked hands from last night’s incident.” His strong condemnation signals an escalation in tensions between U.S. and Iran, which could spread to other countries in the region and has been complicated by ongoing protests in Iraq.

Soleimani led Iran’s Revolutionary Guards’ Quds force and was a household name there, celebrated for helping to defeat Islamic State in Iraq and Syria and countering U.S. influence. State news agency Tasnim said the government has declared three days of public mourning for the veteran of the Iran-Iraq war.