At the heart of the debate, as Biden seeks billions of dollars in additional military aid for Israel’s Gaza operation, are the administration’s own rules for arming foreign nations, which indicate weapons transfers must not take place when the U.S. government assesses that violations of international law are “more likely than not” to occur.

Administration officials, offering the first detailed account of their approach to navigating those guidelines, say they have held extensive discussions with Israeli counterparts to ensure they understand the country’s obligations under international humanitarian law. But they acknowledge the United States is not conducting real-time assessments of Israel’s adherence to the laws of war.

A senior U.S. official, who like other officials spoke on the condition of anonymity to describe sensitive internal discussions, said the administration was unable to make a contemporaneous evaluation of Israel’s compliance in part because officials lack access both to the intelligence Israeli forces use to plan their operations and to commanders’ intentions.

“What we can do from here in real time, feasibly but still rigorously, is to talk about the framework, the legal principles, talk about even some pretty nuanced, particular points of it,” the official said, referring to international norms governing conflict.

In certain instances, administration officials have posed specific questions to the Israelis, as they did following an October strike that killed more than 100 people in a refugee camp. In such cases, Israeli officials have shared information about their targeting and legal analysis, affirming U.S. officials’ conclusion that Israel shares their understanding of its obligations.

“We’re having some very rigorous and at times very tough conversations,” the official said.

Some legal experts say those discussions must be followed by more active monitoring of what occurs with American weapons before additional arms are supplied.

“That’s just the first step,” said Brian Finucane, a former State Department lawyer who now serves as senior adviser with the International Crisis Group. “Even if the U.S. and Israel agree on the same black letter rule, in this case the rules relevant to the conduct of hostilities and targeting, that still leaves open the question of how those rules are being interpreted.”

The heightened focus on American arms supplies comes as Israel intensifies its operations in southern Gaza, with the goal of ensuring that Hamas can never again mount the kind of assault that killed at least 1,200. Biden has vowed unqualified support to Washington’s closest Middle East ally, which for decades has ranked as the top recipient of U.S. security aid. This week, his administration took the unusual step of invoking an emergency declaration to expedite a sale of tank rounds to Israel despite mounting congressional concerns.

American-made arms have played a central role in the war. In the first month and a half, Israel dropped more than 22,000 guided and unguided bombs on Gaza that were supplied by Washington, according to previously undisclosed intelligence figures provided to Congress. And during that time, the United States has transferred at least 15,000 bombs, including 2,000-pound bunker busters, and more than 50,000 155mm artillery shells.

Gaza, meanwhile, is gripped by humanitarian crisis as millions seek shelter from Israeli airstrikes and a ground offensive that have turned to rubble vast swaths of the Palestinian enclave. The United Nations has warned of catastrophe if shipments of vital supplies are not increased significantly.

The situation presents a worsening dilemma for Biden, who has vowed absolute support for Israel’s security but has also promised to put human rights at the center of U.S. foreign policy.

His administration has established a new system at the Pentagon for curbing civilian deaths in U.S. military operations and launched a separate initiative at the State Department to track harm caused by allies employing U.S. arms. Last year, the United States endorsed a global declaration designed to curb the use of explosive weapons in urban areas.

The suffering in Gaza has prompted unusual public admonitions from senior officials including Defense Secretary Lloyd Austin and Secretary of State Antony Blinken, who has pressed Israeli officials to proceed with greater caution.

Since fighting resumed after a seven-day cease-fire on Dec. 1, administration officials have lamented Israel’s use of powerful bombs near densely populated areas — a practice U.S. officials had urgently discouraged in private conversations and meetings in Israel, two U.S. officials said.

“There does remain a gap between exactly what I said when I was there, the intent to protect civilians, and the actual results that we’re seeing on the ground,” Blinken said Thursday.

Israel’s military has sought to outline its procedures for avoiding civilian deaths but has also stressed what it says is the urgent threat facing its citizens. Like the United States, the Israeli military has lawyers who provide commanders with input, attempt to distinguish between combatants and civilians, and provide advance notice of airstrikes in certain situations. Israeli officials say, however, that Hamas embeds militants within civilian sites or in tunnels under them, including hospitals and apartment buildings, and that the group violated the laws of war, including by executing and kidnapping civilians, in its October assault.

U.S. officials who have met with Israeli counterparts in recent weeks cite the process Israeli forces use for calculating the value of individual militant targets and how many civilians are considered acceptable collateral damage. But they also said that Israel’s bar is far higher than the United States’ would be.

Publicly, U.S. officials say it is too soon to judge whether Israel’s conduct complies with the laws of war and that the administration is gathering information to ensure U.S. aid is not being used in ways that violate U.S. laws, a process that could outlast the current conflict.

“This is an extremely challenging space where there is fog of war, where there are challenges to our ability to get proper information,” Mira Resnick, deputy assistant secretary of state in charge of arms transfers, said in a recent interview.

“We don’t have information that would indicate that we have crossed that more-likely-than-not threshold,” she said.

The Biden administration has not declined any request from Israel, officials said, and no cutoff of military assistance is being considered or communicated. And officials stressed that the administration believes it can better influence Israel’s treatment of civilians if it maintains strong backing for the Jewish state.

Still, the senior U.S. official cautioned that the Biden administration’s viewpoint may evolve as the conflict wears on.

“There is never a final answer to this and never a ‘put your pencil down’ moment in looking at what other actors are doing in the world, especially ones we’re providing support to,” the senior official said. “We are always evaluating, and reevaluating and reevaluating, our understanding of what they’re doing with it and our comfort with it.”

The situation recalls earlier moments when Washington has faced difficult decisions about arming allies, as it did in 2016, when the Obama administrations suspended certain arms sales to Saudi Arabia over its repeated bombing of civilian sites in Yemen. The decision followed warnings from State Department lawyers who worried that the United States risked becoming complicit in potential Saudi war crimes in Yemen because it supplies arms to Riyadh.

During the Reagan administration, the United States did suspend arms shipments to Israel over concerns about the use of cluster munitions in neighboring Lebanon.

Annie Shiel, U.S. advocacy director for the Center for Civilians in Conflict, questioned how Biden’s position achieves his stated goal of protecting innocent Gazans.

“On one hand, U.S. officials are saying Israel must do more to protect civilians, while on the other essentially providing a blank check, with no conditions, for how Israel is actually using U.S. assistance,” she said. “Where is the leverage in that?”

It is unclear whether the administration can allay growing concerns in Congress. Democrats have flagged the potential use of American weapons in lethal attacks involving civilians, including an incident in which, according to Amnesty International, U.S. bombs were used in what the group called an unlawful strike that killed 43 civilians in Gaza. Civil society groups have also voiced concerns that U.S. artillery shells could be used in a way that endangers Gazans.

This week, five senators appealed to Biden, condemning the Hamas attack but also decrying the human suffering resulting from Israel’s campaign.

“The risk of violating international law and our own standards increases as Israel uses explosive weapons in densely populated areas,” Sens. Elizabeth Warren (D-Mass.), Jeff Merkley (D-Ore.), Bernie Sanders (I-Vt.), Tim Kaine (D-Va.), and Martin Heinrich (D-N.M.), wrote.

Lawmakers also complain about the administration’s lack of transparency about the weapons provided to Israel, a stark contrast to how it has accounted for aid to Ukraine. A measure introduced this week by Sen. Chris Van Hollen (D-Md.) would require the administration to verify that arms are being used in accordance with international law.

In an interview, Warren said the U.S. government had an obligation to condition assistance on compliance with U.S. and global laws.

“It is critical, she said, “that we follow in real time whether those who receive our aid are, in fact, protecting civilian lives.”