Washington DC - President Joe Biden has stood firmly by Israel amid months of mounting domestic and international outcry over the humanitarian toll in Gaza, but the killing of seven aid workers in an Israeli strike seems to have brought him the closest yet to a breaking point.

"Obviously the World Central Kitchen fiasco has turned the political pressure up," James Ryan of the Middle East Research and Information Project told AFP, referring to the US-based aid group employing the seven workers.

"This turn of events has made it clear... that Israel is not really putting a lot of effort into discriminating between combatants and civilians, to say the least," he said.

Before the killings of the workers, Washington already had growing concern over plans by Prime Minister Benjamin Netanyahu's government for a ground offensive in overcrowded Rafah, fearing that the civilian death toll in Gaza could soar.

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At the end of February, after Israeli forces opened fire on a food aid distribution in northern Gaza, the Biden administration upped pressure on Israel to increase the flow of aid and began its own series of humanitarian air drops.

But it wasn't until Thursday that Biden finally opened the door to conditioning US aid for Israel.

In a tense 30-minute call with Netanyahu, Biden discussed "the need for Israel to announce and implement a series of specific, concrete, and measurable steps to address civilian harm, humanitarian suffering, and the safety of aid workers," the White House said afterward.

It added that Biden "made clear that US policy with respect to Gaza will be determined by our assessment of Israel's immediate action."

For David Makovksy, a senior fellow at The Washington Institute for Near East Policy, the call shows "Washington is no longer willing to maintain its current policy of comprehensive support without rapid changes."

The White House was careful Thursday not to detail what would change if it deemed Israel's actions insufficient.

Nonetheless, the Israeli government announced hours later that it would allow the "temporary" opening of an extra border crossing for aid deliveries at Erez – something it had opposed since the Gaza assault began in October, killing at least 33,037 people.

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On Friday, Israel announced that two officers would be fired after finding a series of "grave mistakes" led to the drone strikes that killed the aid workers.

Biden, who was asked as he left the White House on Friday whether he had threatened to stop military aid to Israel, replied: "I asked them to do what they're doing."

US Secretary of State Antony Blinken welcomed the announcements by Israel, but said "the real test is results and that's what we're looking to see in the coming days and in the coming weeks."

If Washington ultimately views Israel's response as insufficient, Ryan said, it may result in "some conditions or revocations of arms transfers."

The United States provides billions of dollars in military aid to Israel, and while the Biden administration may consider some conditioning, there is little doubt it would threaten a full stop.

However, in a sign of the rising anger among many in Biden's party, more than 30 Democratic members of the US House of Representatives called in a letter Friday for a full halt to the transfer of offensive weapons.

"In light of the recent strike against aid workers and the ever-worsening humanitarian crisis, we believe it is unjustifiable to approve these weapons transfers," said the letter, which was signed by several party leaders, including former House Speaker Nancy Pelosi.

Ryan also noted that the Biden administration could react with "more political pressure – especially at the UN, where the issue of Palestinian membership is going to be before the Security Council within a month or so."

The Palestinians this week officially relaunched their bid to become a full UN member state, hoping for a Security Council vote this month.

Though the Biden administration supports an independent Palestinian state, the State Department said it "should be done through direct negotiations through the parties... and not at the United Nations."