"We have to be prepared for what could be a significant set of attacks," National Security Council spokesman John Kirby told reporters.

"We share the same concerns and expectations that our Israeli counterparts have with respect to potential timing here -- could be this week," added Kirby.

Kirby's comments came as the United States is rushing an aircraft carrier strike group and a guided missile submarine to the region in a show of support for Israel.

Iranian President Masoud Pezeshkian said Monday his country has "the right to respond" to any aggression.

"While emphasising diplomatic solutions to issues, Iran will never give in to pressure, to sanctions and to bullying and considers it has the right to respond to aggressors in accordance with international norms," Pezeshkian said according to a statement published by official news agency IRNA following a phone call with German Chancellor Olaf Scholz.

Iran and Lebanon's Hezbollah group have vowed to avenge last month's assassinations of Hamas political chief Ismail Haniyeh and Hezbollah commander Fouad Shukr.

Iran's Supreme Leader Ayatollah Ali Khamenei has reportedly ordered a direct strike on Israel in response to the assassination of Haniyeh in Tehran. 

Iran's then-acting Foreign Minister Ali Bagheri Kani told AFP in an interview that Israel committed a costly "strategic mistake" by killing Hamas political leader Ismail Haniyeh in Tehran last week.

Bagheri accused Israel of seeking "to expand tension, war, and conflict to other countries," while asserting it was not in a position to fight Iran.

Meanwhile, Hezbollah chief Hassan Nasrallah warned that the group was bound to respond to Israel's killing of its top military commander, saying his death and that of the Hamas leader "crossed" red lines.

"You do not know what red lines you crossed," he said, addressing Israel during a speech broadcast at top commander Fuad Shukr's funeral. "The enemy, and those who are behind the enemy, must await our inevitable response."

Biden called the leaders of France, Germany, Italy and Britain on Monday to discuss the growing tensions, the White House added.

The five leaders issued a joint statement urging Iran to "stand down" its threats of retaliating against Israel.

"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," they said.

The call was "largely for all the leaders to repeat what they have said before in terms of reaffirming Israel's defence" and to "send a strong message that we don't want to see any rise in violence, any attacks by Iran or its proxies."

The leaders also called for a ceasefire in Gaza, with difficult talks set for Thursday on halting the genocidal Israeli war that started on October 7.

Israel has so far killed at least 39,897 people in Gaza, mostly women and children.

* This story has been edited by Ahram Online.