Exclusive: U.S. fears Iran's response to Israeli strike would be mass casualty event

White House envoy Steve Witkoff privately warned top Senate Republicans last week that Iran could unleash a mass casualty response if Israel bombs their nuclear facilities, according to a U.S. official and a source with direct knowledge.

Why it matters: With Israel preparing to strike Iran quickly if Witkoff's diplomacy fails this weekend, President Trump's envoy sounded the alarm about what would come next. Iran's retaliation could overwhelm Israel's defenses and cause heavy damage, he said. State of play: Witkoff is preparing for a sixth round of nuclear talks with Iran on Sunday. It could be the last.

Behind the scenes: Last Thursday, Witkoff held a closed-door briefing on Iran with a group of Republican senators. Among them were Sens. Risch (R-Idaho), Graham (R-S.C.), Barasso (R-Wyo.), and Ricketts (R-Nebr.), the source with knowledge said.

Witkoff told the Senators that military strikes by Israel are on the table if no agreement is reached.
He then brought up Iran's ballistic missile capabilities. The U.S. is concerned Israel's air defenses would not be able to handle an Iranian response involving hundreds of missiles, the sources said.
Such an attack, Witkoff told the group, could cause massive casualties and damage.
Witkoff also raised concerns about Iran's ballistic missile arsenal during a speech in New York on Wednesday, calling it "as big of an existential threat" for Israel as Iran's nuclear capabilities.
Zoom in: According to U.S. intelligence estimates, Iran has 2,000 ballistic missiles with warheads that can carry 2,000 pounds of explosives or more, a U.S. official said. Israel is within range for many of these missiles.

On Wednesday, Iran's minister of defense claimed Iran developed a new ballistic missile with a 4,000-lb. warhead.
The U.S. official said that since the previous Iranian missile strike on Israel, in Oct. 2024, Iran has significantly increased production of ballistic missiles to around 50 per month.
The official said the Iranian goal is to produce more ballistic missiles than the number of missile defense interceptors Israel has.
State of play: CENTCOM commander Gen. Erik Kurilla is expected to arrive in Israel on Saturday for talks with senior political and military officials regarding Iran, Israeli officials say. He's also expected to visit other countries in the region.

Testifying Tuesday before the House Armed Services Committee, Kurilla said an Israeli strike on Iran would expose U.S. forces in the Middle East to the threat of Iranian retaliation.
The U.S. is evacuating non-essential staff from its Baghdad embassy and family members of military personnel from several bases in the Gulf. The State Department imposed security restrictions Thursday on U.S. diplomats and their families in Israel.
The other side: Iran's President Masoud Pezeshkian said Thursday that Iran will not agree to stop enriching uranium, as Trump has insisted it must.

"If they destroy our (nuclear facilities) with a bomb, they will be destroyed. All these capabilities are in our minds, and therefore, whatever they do, we will rebuild," he said.
Islamic Revolutionary Guard Corps commander Gen. Hossein Salami said Thursday that Iran is ready for war and has already determined the targets it will attack.
He stressed Iran's response to any Israeli attack will be "more painful and more destructive" that the two massive missile strikes against Israel last year.
On Thursday, Iran also announced new military exercises it said would "focus on the movements of the enemy."
Between the lines: The fact that two previous Iranian missile attacks ended with limited damage and few casualties gave Israeli officials confidence they could deal with similar attacks in the future.

What to watch: Witkoff will meet Iranian Foreign Minister Abbas Araghchi on Sunday in Oman to discuss Iran's response to the U.S. nuclear deal proposal.

The meeting could determine whether diplomatic efforts continue, or a military operation begins.
Israel's security cabinet will convene Thursday to discuss the crisis.
Witkoff spoke Wednesday with Netanyahu about the rising tensions, and will meet his top adviser Ron Dermer on Friday ahead of his trip to Oman, along with Mossad director David Barnea, according to an Israeli official.
The latest: Araghchi said a resolution condemning Iran passed Thursday by the International Atomic Energy Agency could complicate the nuclear talks.