US Defense Secretary Lloyd Austin has confirmed reports that the United States paused a weapons shipment to Israel, as President Joe Biden’s administration faces growing pressure to condition aid to the top US ally amid the war in Gaza.

Testifying before a US congressional subcommittee on Wednesday, Austin said the Biden administration had paused “one shipment of high payload munitions” amid concerns about the Israeli military’s push to invade the southern Gaza city of Rafah.

“We’ve been very clear … from the very beginning that Israel shouldn’t launch a major attack into Rafah without accounting for and protecting the civilians that are in that battlespace,” Austin told US lawmakers.

“We’ve not made a final determination on how to proceed with that shipment [of weapons],” the Pentagon chief added, noting that the transfer is separate from a supplemental aid package for Israel that was passed in late April.

“My final comment is that we are absolutely committed to continuing to support Israel in its right to defend itself.”

Israel’s ambassador to the United Nations, Gilad Erdan, responded to the shipment pause by saying the US decision was “very dissapointing”.

“[US President Joe Biden] can’t say he is our partner in the goal to destroy Hamas, while on the other hand delay the means meant to destroy Hamas,” Erdan said.

Al Jazeera’s Kimberly Halkett, reporting from the White House on Wednesday, said the shipment included 1,800 bombs each weighing about 900kg (2,000lbs) and another 1,700 bombs each weighing 226kg (500lbs).

“There has been, leading up to this delay, significant concerns on the part of not only student protesters across the United States but also within the president’s own party … about how these weapons are being used,” Halkett said.

US Senator Bernie Sanders welcomed the Biden administration’s pause on the weapons transfer, but said it “must be a first step”.

“The US must now use ALL its leverage to demand an immediate ceasefire, the end of the attacks on Rafah, and the immediate delivery of massive amounts of humanitarian aid to people living in desperation,” Sanders said in a statement. “Our leverage is clear. Over the years, the United States has provided tens of billions of dollars in military aid to Israel.”

The Biden administration has faced months of criticism over its “iron-clad” support for Israel amid the Gaza war, which has killed more than 34,000 Palestinians and plunged the enclave into a dire humanitarian crisis.

But Washington has largely continued to provide military and diplomatic backing to Israel as the war grinds on.

Israel stepped up its bombardment of Rafah on Monday, killing dozens of people after ordering about 100,000 residents in the city’s eastern areas to evacuate.

Israeli troops also stormed the Palestinian side of the Rafah border crossing between the Gaza Strip and Egypt, which serves as a major gateway for humanitarian aid.

Yet despite continuing to say it has concerns for the fate of the more than 1.5 million Palestinians sheltering in Rafah, the US Department of State this week sought to play down the recent moves by the Israeli army.

“This military operation that they launched last night was targeted just to [the] Rafah gate,” US State Department spokesman Matthew Miller said on Tuesday.

“It wasn’t an operation in the civilian areas that they had ordered to be evacuated. So we will continue to make clear that we oppose a major military operation in Rafah.”

Human rights advocates have urged the US to do more to pressure the country to end its war on Gaza, however, and President Biden faces mounting protests — including on US college campuses — over his stance.

A new poll released on Wednesday also suggested a growing disconnect between Biden and his Democratic Party base, which could pose a challenge as he campaigns for re-election in November.

The poll by Data for Progress, in collaboration with news website Zeteo, suggested that 56 percent of Democrats believed Israel was committing “genocide” in the besieged Palestinian territory.

It also found that seven in 10 American voters — and 83 percent of Democrats — also support a permanent ceasefire in Gaza.

Hasan Pyarali, the Muslim Caucus chairman for College Democrats of America, the university arm of the Democratic Party, told Al Jazeera last week that many young people have signalled they will not vote for Biden in the upcoming election.

“It’s not just good policy to oppose the genocide; it’s good politics,” he said.

The United Nations defines genocide as “acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group”, including killings and measures to prevent births.

In January, the International Court of Justice — the UN’s top court — acknowledged there was a plausible risk of genocide in Gaza and ordered Israel to take “all measures within its power” to prevent genocidal acts against Palestinians.

Israel has rejected the accusation that it is committing genocide.