Israel-Gaza updates: 22, including 18 children, reported dead in Rafah strikes

Israel Defense Forces spokesman Rear Adm. Daniel Hagari said Israel focused too intensely on the Iranian nuclear threat at the expense of its ballistic threat.

A senior U.S. official told ABC News the U.S. also relied too heavily on the misguided conception that Iran's Supreme Leader Ayatollah Ali Khamenei was cautious and would never order a direct attack on Israel, and that this weekend’s attack and the general U.S. assessment of Iran now requires study and reassessment.

Sima Shine, a former head of the Iran desk at Israeli espionage agency Mossad, also said Israel’s assessment was wrong, and said "the rules of the game" have changed. A huge barrage of missiles was considered possible, but highly unlikely, Shine said.

Shine said any Israeli response under the new conception requires the assumption that Iran will follow up with its threat of another salvo of missiles. That said, Shine believes that Iran and the supreme leader do not want a full-scale war because it would be unpopular in Iran and the U.S. could get involved.

-ABC News’ Matt Gutman

Treasury Secretary Janet Yellen is warning Iran that the U.S. "will not hesitate" to impose new sanctions in response to Iran's "unprecedented attack" on Israel.

"Treasury will not hesitate to work with our allies to use our sanctions authority to continue disrupting the Iranian regime’s malign and destabilizing activity," Yellen is expected to say at a Tuesday press conference. "The attack by Iran and its proxies underscores the importance of Treasury's work to use our economic tools to counter Iran's malign activity."

Yellen's message follows President Joe Biden's Sunday meeting with the G7 nations, during which the leaders discussed a coordinated effort on sanction measures.

-ABC News’ Justin Ryan Gomez

Prime Minister Benjamin Netanyahu's war cabinet is expected to meet again on Tuesday to consider an Israeli response to Iran's weekend attack.

"We are closely assessing the situation. We remain at our highest level of readiness," Herzi Halevi, chief of staff of the Israel Defense Forces, said on Monday. "Iran will face the consequences for its actions."

Halevi added Israel would "choose our response accordingly."

-ABC News' Morgan Winsor and Somayeh Malekian

United Nations officials called on Tuesday for Israel and Iran to de-escalate their conflict, saying the retaliatory military attacks “violate the right to life and must cease immediately.”

“All countries are prohibited from arbitrarily depriving individuals of their right to life in military operations abroad, including when countering terrorism,” the U.N.’s Office of the High Commissioner for Human Rights said in a press release, quoting U.N. officials described as "experts."

The retaliatory strikes by both countries may constitute the “international crime of aggression by civilian and military leaders responsible,” those officials said, according to the statement.

-ABC News' Kevin Shalvey