The leaders of the U.S., U.K., France, Germany and Italy called on Iran to "stand down" and expressed their support for Israel's defense "against Iranian aggression" during a call on Monday, according to a joint statement released by the White House.

“We called on Iran to stand down its ongoing threats of a military attack against Israel and discussed the serious consequences for regional security should such an attack take place," the statement said.

The leaders also expressed their support for ongoing efforts to reach a cease-fire and hostage release deal in Gaza, according to the statement.

-ABC News' Justin Gomez

As the Middle East continues its uneasy wait for Iran's response to the killing of Hamas political leader Ismail Haniyeh, the State Department is pressing on with its high-stakes diplomatic campaign to constrain military action from Tehran amid fresh waves of uncertainty.

"We continue to work diplomatically to prevent any major escalation in this conflict,” deputy spokesperson Vedant Patel told reporters Monday. "We obviously don't want to see any kind of attack or response happen in the first place."

Multiple officials within the State Department said they are still cautiously optimistic that Iran will limit the scope of its retaliation, but that they increasingly expect the country will strike at Israel before Thursday -- the date the Biden administration, along with Egypt and Qatar, set to relaunch hostage/cease-fire deal talks in hopes of bringing Israel and Hamas back to the table for a final push.

A joint statement issued by the mediators last week was designed not only to pressure the parties involved, but as a message to Iran that an agreement was in the offing meant to persuade the country against military action that could scuttle a deal, according to an official.

National Security Council spokesperson John Kirby also publicly warned Monday that Iranian military action could impede progress at a critical point in the negotiations.

However, Hamas' earlier announcement that it would not participate in the round of negotiations and public infighting among top Israeli officials have cast significant doubt over whether the Thursday meeting will even happen -- undercutting the administration's intended message to Tehran.

At the podium Monday, Patel said mediators "fully expect talks to move forward as they should" in order to "bring this deal to conclusion."

He declined to say whether Hamas or Israel was the bigger impediment.

"I'm not going to color it one way or the other," Patel said while noting that "the prime minister of Israel immediately welcomed this initiative and confirmed that the Israeli team will be there, and they'll be prepared to finalize the details of implementing the deal."

-ABC News' Shannon K. Kingston

A United States assessment shows a retaliatory attack by Iran and its proxies against Israel could be launched "this week," the White House said Monday.

"We share the same concerns and expectations that our Israeli counterparts have with respect to potential timing here. Could be this week," National Security Council spokesperson John Kirby told reporters. "We're continuing to watch it very, very closely."

Kirby said it's difficult to ascertain what a potential attack could look like at this time but that "we have to be prepared for what could be a significant set of attacks."

Meanwhile, Israel Defense Forces spokesperson Daniel Hagari told reporters on Monday that Israel remains "on high alert."

"We take seriously the threats of our enemies and that is why we are on high alert offensively and defensively," he said.

Hagari said the IDF will "work hard to give the public time to get organized."

-ABC News' Justin Gomez

Israeli Defense Minister Yoav Gallant appeared to take a swing at Israeli Prime Minister Benjamin Netanyahu during a Knesset committee meeting about Israel’s response to ongoing cross-border tensions with Lebanese militant group Hezbollah.

"I hear all the heroes with the war drums, the 'absolute victory' and this gibberish," Gallant reportedly said, alluding to Netanyahu's slogan through the war, according to Israeli media.

Netanyahu's office released a statement shortly after, saying Gallant too is bound by the policy of "absolute victory."

"When Gallant adopts the anti-Israel narrative, he hurts the chances of reaching a deal for the release of the abductees," the Israeli Prime Minister's Office said. "He should have attacked [Hamas leader Yahya] Sinwar, who refuses to send a delegation to the negotiations, and who was and remains the only obstacle to the kidnapping deal."

National Unity Party Chairman Benny Gantz warned about internal divisions in Israel during an address on Monday.

"If we don't come to our senses, there will be a civil war here," Gantz said.

He said there have been "heroes," from soldiers to volunteers, in the wake of the Oct. 7 attack, but also "leadership that dissolves, networks that poison the well from which we live."

"The patriotic Israeli majority should stop the hatred and make amends," Gantz said.

-ABC News' Jordana Miller