WASHINGTON -- The United States has withheld one shipment of near-term weapons aid to Israel due to "serious concerns" that a looming large-scale military operation in Rafah by Israeli forces would cause significant loss of civilian lives, the State Department said on Wednesday.

Matthew Miller, the spokesperson for the State Department, confirmed the halt of security assistance to Israel at a press briefing, telling reporters that the administration of President Joe Biden is reviewing other shipments as the situation in Rafah unfolds.

"We have very serious concerns about a potential Rafah operation. We have concerns about what that would mean for the civilian population," Miller said, adding that such concerns on the US side stemmed from "the way Israel has conducted its operations in the past and what the impact on the civilian population has been."

Miller's confirmation of the delay came days after multiple US media outlets reported on the administration's move, with Axios - which first published such a report - citing a senior administration official as saying that weapons being put on hold included 1,800 2,000-pound bombs and 1,700 500-pound bombs.

"We also have concerns about the impact of any potential operation on the delivery of humanitarian assistance," Miller said, urging Israel to open the Rafah crossing so that humanitarian assistance would continue flowing into Gaza.

The Israel Defense Forces (IDF) said in a statement early Tuesday that its troops "managed to establish operational control of the Gazan side" of the Rafah crossing following a "precise counterterrorism operation" launched overnight against Hamas.

Miller said the IDF's incursion into Rafah was conducted without US approval, adding that Washington considered it to be the "first priority" that the Rafah crossing be kept open.

Earlier on Wednesday, US Defense Secretary Lloyd Austin told senators on Capitol Hill as he testified on the Pentagon's fiscal year 2025 budget proposal that the Pentagon is "going to continue to do what's necessary to ensure that Israel has the means to defend itself."

"But that said, we are currently reviewing some near-term security assistance shipments in the context of unfolding events in Rafah," he added.