The Biden administration’s pause in a shipment of high-payload munitions to Israel reverberated through Capitol Hill on Wednesday, with Democrats largely backing the move as necessary to convince Prime Minister Benjamin Netanyahu’s government to hold off on military actions in the Gaza Strip and Republicans warning Iran might conclude U.S. backing of Israel is no longer ironclad.

The administration last week “paused” one shipment of near-term assistance consisting of 3,500 bombs weighing 2,000 pounds and 500 pounds, said a U.S. official, who was not authorized to be quoted. It was the first known instance of the administration holding back security assistance to Israel in the seven-month Israel-Hamas war.

The Israeli military appears poised to begin a ground assault on the city of Rafah, where some 1.3 million Palestinians — many of them already displaced at least once by the war — are crowded together in unsafe and dire living conditions. Israel is seeking to root out the Palestinian militant group Hamas, which governs Gaza. 

“We are especially focused on the end-use of the 2,000-lb bombs and the impact they could have in dense urban settings as we have seen in other parts of Gaza,” the official added. “We have not made a final determination on how to proceed with this shipment.”

State Department spokesman Matthew Miller said in a Wednesday briefing that the U.S. is concerned about the potential for heavy civilian casualties in Rafah.

“We do not believe Israel has presented a credible humanitarian plan that would account for both the potential harm to civilian life of an operation in such a crowded area or for properly evacuating and caring for the over a million people who were there,” he said.

House Speaker Mike Johnson, R-La., and Senate Minority Leader Mitch McConnell, R-Ky., wrote in letter to President Joe Biden on Wednesday that they were “alarmed” by the pause, saying it “flies in the face of assurances provided regarding the timely delivery of security assistance to Israel.”

Democrats, particularly progressives, have been pressing the administration for months to use its leverage over weapons transfers — the U.S. is by far Israel’s largest supplier of military assistance — to pressure the Netanyahu government to limit civilian casualties in Gaza and to cease blocking humanitarian aid shipments.

Nearly 35,000 Palestinians, most of them civilians, have been killed in the conflict. Parts of the densely packed Gaza are now in a full-blown famine, according to Cindy McCain, the head of the U.N. World Food Program. Israel launched its bombardment of Gaza in response to Hamas’ Oct. 7 terrorist attack on the Jewish country, which killed some 1,200 people.

Sen. Tim Kaine, D-Va., who sits on both the Senate Armed Services and Foreign Relations panels, in a statement said he was pleased with the halt in the weapons delivery and urged the administration to be cautious about further transfers that “could be used in offensive military actions that result in significant civilian casualties.”

“I expect the administration to soon deliver to Congress its report pursuant to National Security Memorandum 20, which will include assessments of whether Israel has used U.S.-funded weapons in compliance with international law or whether Israel has impeded the delivery of humanitarian aid,” Kaine said, referring to the administration’s commitment in a February national security memorandum to report to Congress.

That memo, known as NSM-20, was a bid to stave off an intra-Democratic Senate floor fight over whether to place conditions on security assistance to Israel in the national security supplemental. That law provided billions in Israel-related funding, including $3.5 billion for State Department-managed grants, loans and loan guarantees to help Israel purchase U.S.-made weapons through the Foreign Military Financing program.

The memorandum requires recipients of U.S. security assistance to provide written assurances they are using U.S.-supplied weapons in compliance with U.S. and international laws and to not restrict the delivery of humanitarian aid, It also laid out new reporting commitments to Congress on the matter.

The first report was due Wednesday, but Foggy Bottom said the deadline, which it described as “self-imposed,” would slip a little. Miller said the department expects to deliver it “in the very near future, in the coming days.”

“The national security memorandum says clearly you cannot transfer aid or weapons if Israel is not in compliance with the rules of war, humanitarian law,” Sen. Christopher S. Murphy, D-Conn., who leads the Senate Foreign Relations Middle East subcommittee, said about the weapons pause. “This potential invasion of Rafah would clearly not be in compliance with U.S. and humanitarian law.”

The State Department is also reviewing whether to fulfill requests by Israel for kits that can turn unguided bombs into precision-guided munitions, but those transfers are further down the road, the U.S. official said.

“To be clear: None of these shipments have anything to do with the Israel supplemental appropriations passed last month. All are drawn from previously appropriated funds, some many years ago. We are committed to ensuring Israel gets every dollar appropriated in the supplemental,” the official said, noting the State Department just approved a large tranche of FMF weapons to Israel worth $827 million.

Defense Secretary Lloyd J. Austin III, at a Senate Defense Appropriations Subcommittee hearing, defended the administration’s decision to put the brakes on the high-payload munitions “in the context of unfolding events” in Rafah.

Republicans warn of emboldening Iran

Republicans were quick to blast the administration for sending a signal to the region that U.S. weapons support isn’t always guaranteed.

“You said in response to a question today that no final decision [on the weapons shipment] has been made,” Senate Appropriations ranking member Susan Collins, R-Maine, said to Austin. “I would suggest to you that pausing or delaying the delivery of weapons to Israel is a decision and it’s a decision that most members of Congress would take issue with.”

Added Sen. Jerry Moran, R-Kan.:“Does this not send the wrong message to our ally Israel and embolden Iran and Iranian-backed groups? We should not be signaling to [Israel’s] enemies that our support is conditional.”

Iran is the main government supporter of Hamas and also backs other proxy groups to destabilize the region. 

Miller said Wednesday that the U.S. commitment to Israel’s broader security is unchanged.

“People should be absolutely clear that we are committed to Israel’s security,” he said, noting how the U.S in recent weeks led a multinational effort that successfully intercepted nearly all of the roughly 300 missiles and drones launched at Israeli territory by Iran.

In pausing the bomb shipment, Secretary of State Antony Blinken is using legal authorities provided by Congress under a 1961 law on foreign assistance and a 1976 arms export law to regulate foreign weapons transfers, including ones paid for with congressionally directed appropriations, particularly when there is evidence the intended recipients have engaged in gross human rights violations.

John M. Donnelly contributed to this report.