Fears of escalation into a regional war are top of mind for international leaders who urged restraint, many condemning Iran for its retaliatory strike and expressing solidarity with Israel.

Western allies, such as Germany, Canada and the United Kingdom, had harsh words for the Iranian regime. U.K. Prime Minister Rishi Sunak accused Iran of "sowing chaos in its own backyard" while Canadian Prime Minister Justin Trudeau said that the attack demonstrated "the Iranian regime’s disregard for peace and stability in the region."

Sen. John Fetterman, D-Pa., says he is shocked by President Joe Biden's message to Israel that the U.S. will not participate in operations against Iran after it fired missiles at Israel.

"It’s astonishing that we are not standing firmly with Israel," Fetterman said on CNN Sunday morning. "There should never be any kind of conditions on all that. When a nation can launch hundreds of drones at Israel, I’m not going to be talking conditions, ever."

When asked if he thought Biden's warning to Israeli Prime Minister Benjamin Netanyahu was the right call, he bluntly said: "I don’t agree with that."

"I just think we should follow and have Israel’s back in the situation," he added. "I don’t agree with the president."

HONG KONG — China expressed “deep concern” over the Iranian attack on Israel, describing it as “the latest spillover of the Gaza conflict” and calling for restraint by relevant parties to prevent further escalation.

“China calls on the international community, especially countries with influence, to play a constructive role for the peace and stability of the region,” the Foreign Ministry said in a statement today.

China has sought to serve as a mediator in the Middle East, an increasingly important source of energy imports for the world’s second-largest economy.

Israeli Prime Minister Benjamin Netanyahu, center, convened his war management Cabinet in the Kirya area of Tel Aviv late yesterday.

Former Russian President Dmitry Medvedev said on the messaging platform Telegram that a war between Israel and Iran would hurt Joe Biden’s chance of being re-elected U.S. president.

“What is the difference for Washington between what happened in the Middle East and the events in ‘Ukraine’? America does not want a big war in the Middle East. And it tries to maintain balance and show restraint,” he wrote. 

“Killings in Gaza worsen Biden’s election prospects, and the war between Israel and Iran adds further uncertainty.”

Iran has sought to make clear this morning that it considers its exchange with Israel over, while warning against a response.

Tehran informed the U.S. ahead of time that it would launch a "limited" retaliatory attack against Israel for self-defense, Foreign Minister Hossein Amirabdollahian said in a meeting with foreign ambassadors in the Iranian capital today. He also said that Iran's regional neighbors were given 72 hours notice that the retaliatory strikes were coming.

President Ebrahim Raisi said that his country's armed forces had "taught a lesson to the Zionist enemy." Raisi warned that "any new adventure will be responded with a heavier and regrettable response."

And Tehran said that it had summoned the French, British and German envoys over their reactions to the attack.

Pope Francis addressed the escalating tensions in the Middle East, calling for an end to the worsening violence.

"I make a heartfelt appeal for a halt to any action that might fuel a spiral of violence with the risk of dragging the Middle East into an even greater war conflict," he said, speaking in Italian in public remarks after reciting the Regina Coeli prayer.

"No one should threaten the existence of others," he continued. "Instead, all nations should take the side of peace and help the Israelis and Palestinians to live in two states, side by side, in security."

The pope urged a cease-fire and for hostages to be freed.

"No more war, no more attacks, no more violence," he said. "Yes to dialogue and yes to peace.”

Iran's attack on Israel has caused some air travel disruptions in the region, with Imam Khomeini International Airport in Tehran canceling all flights through at least 6 a.m. local time Monday, according to Iran's semiofficial ISNA news agency. In Israel, a spokesperson for the Israeli Airport Authority said airspace was back open but that travelers should check with airlines to see if their flights had been rescheduled.

At least one airline, Lufthansa, canceled flights in advance of the attack, announcing on Friday that it had suspended flights into Tehran and Beirut through Thursday, April 18. It also suspended flights to and from Tel Aviv; Erbil, Iraq; and Amman, Jordan, through Monday.

Emirates Airlines canceled and rerouted flights immediately following the attack, but said it had resumed operations to and from Jordan, Lebanon and Iraq as of Sunday afternoon local time.

Iran's unprecedented attack on Israel marks a significant moment in history, even if it was intended to be largely symbolic, according to an analyst with geopolitical risk advisory firm Eurasia Group.

"Make no mistake: even if #Iran intended this to be a telegraphed and (it would seem) largely symbolic show of force to restore deterrence, it has attacked Israel with missiles and drones for the first time," tweeted Gregory Brew, an Iran analyst.

"Alea iacta est, folks," he added, using a Latin term for "the die is cast."

Brew wrote that he "did not anticipate Iran would use its own missiles, from within its own territory, against Israel directly."

"A major milestone," Brew tweeted.