Washington halted the load of 1,800 2,000-lb (907 kg) bombs and 1,700 500-lb (226 kg) bombs after Israel had not "fully addressed" US concerns about a major ground operation, a senior administration official said.

News of the move comes as the White House blasted the "unacceptable" closure of the Rafah border crossing with Egypt after Israel sent tanks in earlier Tuesday -- while holding out hope for a ceasefire with Hamas.

The pausing of weapons marks the first time that Biden has acted on the warning that he gave Prime Minister Benjamin Netanyahu in April that US policy on Gaza would depend on how Israel treated civilians.

The US official said Biden's administration made the decision on the weapons when it appeared Israel was on the verge of a major ground operation into Rafah, which Washington has strongly opposed as more than a million people are sheltering there.

Israeli and US officials had been discussing alternatives but "those discussions are ongoing and have not fully addressed our concerns," the senior US official said, speaking on condition of anonymity.

"As Israeli leaders seemed to approach a decision point on such an operation, we began to carefully review proposed transfers of particular weapons to Israel that might be used in Rafah. This began in April."

The US official said Washington was "especially focused" on the use of the heaviest 2,000-lb bombs "and the impact they could have in dense urban settings as we have seen in other parts of Gaza."

"We have not made a final determination on how to proceed with this shipment," the official said.

The US State Department is still reviewing other weapons transfers, including the use of precision bomb kits known as JDAMs, added the official.

The Biden administration is due to deliver a first-of-its-kind formal verdict this week on whether the airstrikes on Gaza and restrictions on delivery of aid have violated international and US laws designed to spare civilians from the worst horrors of war.

The administration agreed in February at the insistence of Democrats in Congress to look at whether Israel has used US-provided weapons and other military assistance in a lawful manner.

Additionally, under the same agreement, it must tell Congress whether it deems that Israel has acted to “arbitrarily deny, restrict, or otherwise impede, directly or indirectly,” delivery of any US-supported humanitarian aid into Gaza for starving civilians there.

State Department spokesman Matthew Miller told reporters Tuesday that the department was trying to meet the Wednesday deadline for completing the review but “it’s possible it slips just a little bit.”

A decision against Israel would further add to pressure on Biden to curb the flow of weapons and money to Israel’s military.

The White House said earlier that Israel had promised the military thrust into Rafah earlier Tuesday was a "limited operation" and not the feared full-scale invasion.

 'Absurd demands' 

While facing a difficult reelection battle in November against Republican rival Donald Trump, Biden has come under growing domestic pressure over his support for Israel, with widespread protests at US universities leading to police crackdowns and thousands of arrests.

Republicans have also piled on the political pressure, and US House Speaker Mike Johnson -- who has repeatedly claimed the protests are anti-Semitic -- criticized on Tuesday any move to limit military aid.

"Just when we thought it was only university presidents who are caving into the absurd demands of the pro-Hamas students, the president himself has now reportedly halted ammunition shipments to Israel," Johnson said to reporters ahead of the official US announcement.

Johnson said it was "undermining" a huge aid package with military support for Israel and Ukraine that Congress passed after months of delays in April -- although the US official said the halted shipment was not linked to that package.

Biden's move does however turn the screw on Israel to end the seven-month war and to take more action on the humanitarian situation in Gaza.

"The crossings that have been closed need to be reopened, it is unacceptable for them to be closed," White House Press Secretary Karine Jean-Pierre told a briefing earlier Tuesday over Israel's seizure of the Rafah crossing.

Meanwhile, ceasefire talks involving Israel, Hamas, the United States, Qatar and Egypt resumed on Tuesday.

"A close assessment of the two sides' positions suggests that they should be able to close the remaining gaps, and we're going to do everything we can to support that process," US National Security Council spokesman John Kirby said.