President Donald Trump threatened to attack multiple Iranian sites if the Middle Eastern nation strikes any American people or assets in a series of tweets Saturday, as Iran mourns the loss of a top leader, Maj. Gen. Qassem Soleimani, who was killed in a US airstrike in Iraq on Friday.

Iranian Supreme Leader Ayatollah Khamenei promised “harsh revenge” for Soleimani’s death, who was considered the second-most powerful Iranian official, after a three-day mourning period.

If Iran follows through on that promise, the US will target 52 sites within the country “some at a very high level & important to Iran & the Iranian culture,” Trump wrote. “Those targets, and Iran itself, WILL BE HIT VERY FAST AND VERY HARD. The USA wants no more threats!”

The Trump administration already faces questions about the legality of its attack on Soleimani, and Sunday, Iranian officials accused Trump of threatening to carry out war crimes against the country.

“Having committed grave breaches of int’l law in Friday’s cowardly assassinations, [Trump] threatens to commit again new breaches of JUS COGENS,” Javad Zarif tweeted, with “jus cogens” referring to the norms of international law. “Targeting cultural sites is a WAR CRIME.”

Zarif went on to compare Trump to ISIS, which also targeted cultural sites during its offensive in Syria and Iraq, a comparison that was echoed by Iran’s Minister of Communications and Information Technology Mohammad Javad Azari-Jahromi, who tweeted Sunday, “Like ISIS, Like Hitler, Like Genghis! They all hate cultures. Trump is a terrorist in a suit.”

On CNN’s State of the Union, Secretary of State Mike Pompeo defended Trump’s threats of strike on cultural targets, calling them “consistent with the rule of law” and promising that the Trump administration will “do the things that are right.” He did not specify how attacking cultural or civilian targets would be consistent with international law.

Ahead of Pompeo’s television appearance, Trump doubled down on his threats on Twitter, writing that the US military is the “biggest and by far the BEST in the World! If Iran attacks an American Base, or any American, we will be sending some of that brand new beautiful equipment their way...and without hesitation!” He also said that if Iran retaliates as promised, “we will hit them harder than they have ever been hit before!”

According to a CNN interview with Khamenei’s military adviser, Maj. Gen. Hossein Dehghan, Iran does, in fact, plan to attack US military installations in retaliation for the assassination of Soleimani. Dehghan also said that Iran will work to strike when US leaders least expect it, saying, “Our reaction will be wise, well considered and in time, with decisive deterrent effect.”

Soleimani’s killing was a game-changer in a longstanding escalation of tensions between the US and Iran that began with Trump’s withdrawal from the Iran nuclear deal in 2018 and continued with a series of crippling sanctions over the country’s missile program and support for terrorism.

As Vox’s Dylan Scott has explained, Iran has met each of these US actions with increasingly bold responses:

Iran has responded with a series of provocative acts, from attacking container ships in the Persian Gulf to drone strikes on Saudi oil facilities. Through its proxy militias, it has also struck directly at American assets, last week killing a US contractor near the Iraqi city of Kirkuk, which in turn lead to US strikes on Iranian militias and then violent protests at the US embassy in Baghdad led by those Iranian-backed militias.

These escalating tensions, further heightened by the killing of Soleimani, Trump’s recent threats, and Iran’s promises of revenge, mean the US and Iran have taken another major step toward a conflict that national security expert Ilan Goldenberg told Vox’s Alex Ward would be “so much worse than Iraq.”

Fortunately, experts have said that all-out warfare is unlikely (though still possible) given that neither country is likely to be eager to jump into armed conflict. But Iranian officials have deemed Soleimani a “martyr” whose death requires retaliation.

Soleimani’s death has united Iranians across the political spectrum as they undergo three days of national mourning to commemorate the leader deemed a hero. State news outlets allotted their entire broadcasts Friday to commemorate him, and comedy films and concerts were postponed, Al Jazeera reported.

Tens of thousands of mourners filled the streets of Ahvaz and Mashhad Sunday, where Soleimani’s remains were transported for public processions.

Mourners in Baghdad Saturday chanted “Death to America” and “We will take our revenge,” according to the Associated Press.

And beyond tit-for-tat actions between Iran and the US, Soleimani’s killing seems poised to have other, far-reaching repercussions.

Iran’s Foreign Ministry said Sunday morning Iranian officials are planning an even larger retreat from the broken nuclear deal than originally planned. The country has already begun stockpiling uranium and exceeding enrichment limits imposed by the agreement, and Sunday afternoon Iranian officials said the country will no longer abide by any of the commitments outlined in the deal.

In Iraq too, where Soleimani’s death took place, leaders hope to make radical changes to the current state of affairs. That country’s prime minister recommended parliament order US troops to leave the country over concerns the US violated Iraqi sovereignty in its assassination. Sunday, lawmakers quickly voted for a resolution to do just that, in a move that could completely alter the US-Iraq relationship and fundamentally change the fight against ISIS.

Even before the vote and Iran’s announcement on its nuclear plans, experts agreed the escalating tensions would cause further instability in the Middle East. Just days after Soleimani was killed, that instability seems all the more certain.