Baghdad, Iraq CNN  — 

Iranian President Hassan Rouhani said Saturday the United States committed a “grave mistake” in killing Iranian commander Qasem Soleimani and US President Donald Trump responded on Twitter, writing that if Tehran attacks American assets, the US will strike “very hard and very fast.”

The US has a list of 52 Iranian targets, Trump tweeted. The number was chosen to match the number of hostages taken in the 1979 takeover of the US Embassy, he said.

Rouhani said earlier Americans would face consequences for killing Soleimani “not only today, but also in the coming years.”

Rouhani’s remarks came on the same day mourners in neighboring Iraq had chanted “Death to America” at a funeral procession for Soleimani and an Iraqi militia leader who died with him in a US airstrike in Iraq early Friday.

“The Americans did not understand what grave mistake they committed,” Rouhani said while visiting the house of Soleimani’s family in Tehran, according to a statement his office released.

Iranian Foreign Minister Javad Zarif seemed to suggest a goal of any Iranian retaliation, tweeting Saturday: “End of US malign presence in West Asia has begun.”

In a phone conversation with Turkish President Recep Tayyip Erdogan, Rouhani said Iran’s neighbors in the region should work in unison against the US presence there or the whole area will be in jeopardy, Iranian state news agency IRNA reported.

Major escalation

The strike killed Soleimani at Baghdad International Airport, along with Abu Mahdi al-Muhandis, deputy head of the Iran-backed Iraqi Popular Mobilization Forces. At least six people were killed in the strike, an Iraqi security source told CNN on condition of anonymity.

Maj. Gen. Ismail Qaani, who served for years alongside Soleimani, has been appointed his replacement.

It marks a major escalation in regional tensions that have pitted Tehran against Washington and its allies in the Middle East.

Trump on Friday said he ordered the death of Soleimani, one of Iran’s most powerful men, to stop a war, not start one, as tensions between the two nations were already escalating.

Trump said Soleimani was plotting “imminent and sinister attacks” on Americans.