By Mike Stone, Steve Holland and Phil Stewart

WASHINGTON (Reuters) -U.S. President Joe Biden's administration paused a shipment of weapons to Israel last week in opposition to apparent moves by the Israelis to invade the southern Gaza city of Rafah, a senior administration official said on Tuesday.

Biden has been trying to head off a full-scale assault by the Israelis against Rafah, where hundreds of thousands of Palestinian have sought refuge from combat elsewhere in Gaza.

The official, speaking on condition of anonymity, said that as Israelis leaders seemed to approach a decision on a Rafah incursion, "we began to carefully review proposed transfers of particular weapons to Israel that might be used in Rafah" beginning in April.

"As a result of that review, we have paused one shipment of weapons last week. It consists of 1,800 2,000-lb bombs and 1,700 500-lb bombs," the official said.

"We are especially focused on the end-use of the 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.

Four sources said the shipments, which have been delayed for at least two weeks, involved Boeing-made Joint Direct Attack Munitions, which convert dumb bombs into precision-guided ones, as well as Small Diameter Bombs.

The delay comes at a time when Washington is publicly pressuring Israel to postpone its planned offensive in Rafah until after it has taken steps to avert civilian casualties.

The White House and Pentagon declined comment.

Israeli forces on Tuesday seized the main border crossing between Gaza and Egypt in Rafah, cutting off a vital route for aid into the tiny enclave.

Without addressing whether there had been a holdup in arms shipments, White House press secretary Karine Jean-Pierre reaffirmed that Washington's commitment to Israel's security was "ironclad."

Still, when asked about the reports on the arms holdsups, she added: "Two things could be true, in the sense of having those conversations, tough, direct conversations with our counterparts in Israel ... in making sure citizens lives are protected ... and getting that commitment."

The Pentagon said on Monday that there had not been a policy decision to withhold arms from Israel, America's closest Middle East ally.

Still, the delays appeared to be the first since Biden's administration offered its full support to Israel following Hamas' Oct. 7 attack, which killed about 1,200 people with about 250 others abducted, of whom 133 are believed to remain in captivity in Gaza, according to Israeli tallies.

Israel's campaign to destroy Hamas has led to a seven-month-long military campaign that has killed a total of 34,789 Palestinians, most of them civilians, the Gaza Health Ministry said.

The conflict has also left many of Gaza's 2.3 million people on the brink of starvation and sparked protests in the U.S. demanding that universities and Biden withdraw support for Israel - including the provision of weaponry.

A senior Israeli official, speaking to Reuters on condition of anonymity, did not confirm any specific hold-up in arms supplies but appeared to take the reports in stride: “As the prime minister has already said, if we have to fight with our fingernails, then we’ll do what we have to do.”