Iran rejected calls by the U.S. and Arabs to measure its response to the assassination of Hamas leader Ismail Haniyeh in Tehran.
This comes after Iranian officials announced they were investigating the security breaches that led to the attack.
Iranian leaders have pledged to retaliate, telling Arab diplomats on Saturday that it does not matter to them if the response led to a war, according to The Wall Street Journal.
The U.S. requested that partner governments in Europe and elsewhere convey a message to Iran not to escalate tensions, stressing that any significant strike would illicit a response and that new Iranian President Masoud Pezeshkian's efforts to improve relations with the West would be better received if Iran shows restraint, according to the report.
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U.S. officials said in its message that they were also urging Israel to deescalate tensions.
Nobody has claimed responsibility for the attack on Haniyeh, but Iran and Hamas have accused Israel of carrying out the strike.
Jordan's foreign minister, Ayman Safadi, visited Tehran on Sunday to talk about how to scale back tensions, according to The Wall Street Journal report. In April, Jordan intercepted projectiles that had entered its airspace headed toward Israel during Iran's drone and missile attack on the Jewish State, and Jordan suggested it would do the same again.
"Anyone who wants to violate our skies, we will face that," Safadi said last week. "Jordan will not be a battlefield. We are exposed to many consequences."
Israel said it was prepared to defend against and respond to any retaliatory strike.
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"Israel is now in a multifront war against the Iranian axis of evil," Israeli Prime Minister Benjamin Netanyahu said on Sunday. "We are ready for any scenario, either defensive or offensive. I repeat to our enemies: We will respond and exact a heavy price for any act of aggression against us, from any arena."
Haniyeh, who was in Tehran to attend the inauguration of Iran's new president, was killed by a bomb while staying in an Islamic Revolutionary Guard Corps guesthouse, according to The Wall Street Journal.
Iran and Hamas have disputed that a bomb killed Haniyeh, claiming instead that he was struck by a missile. Israel has not publicly commented on the killing.
Iran's judiciary deputy chief, Sadeq Rahimi, said Saturday that the country’s public prosecutor opened an investigation into Haniyeh's assassination and ordered to identify and arrest anyone who was negligent or knowingly worked with Israel in the killing, according to the local Fars news agency, which is managed by Iran’s Islamic Revolutionary Guard Corps.
"Whether Israel has used infiltratory elements, human agents and spies, or it has committed this crime directly are under investigation," Rahimi said, according to Mehr news, which is also close to the IRGC.
Politicians in Tehran have said the attack exposed intelligence failures by Iran’s security agencies. "There are gaps and contaminations in the security information system of the country," Ahmad Bakhshaish Ardestani, a member of the foreign affairs and national security commission in Iran’s Parliament, told Iran's Didban news site.
Israel has put its military on high alert, as U.S. officials have worked to have military assets and regional partners prepared to stop an attack.
"We are preparing for every possibility, just as we did in advance of April 13 when Iran attacked Israel and the United States and a coalition of our partners and allies worked with Israel to defeat that attack," U.S. deputy national security adviser Jon Finer told ABC News on Sunday.
The Wall Street Journal contributed to this report.
