WASHINGTON - The Biden administration said on Tuesday it was closely monitoring the probe into a deadly Israeli airstrike it called tragic, but that the recent deaths in Rafah didn't constitute a major ground operation there that crosses any US red lines.

"The Israelis have said this is a tragic mistake," National Security Council spokesman John Kirby told reporters at the White House, when asked about whether the events over the weekend qualified as the type of "death and destruction" US officials have warned could result in the withholding of more aid to Israel.

The US doesn't have "a measuring stick here or a quota," Kirby said.

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"We've also said we don't want to see a major ground operation in Rafah that would really make it hard for the Israelis to go after Hamas without causing extensive damage and potentially a large number of deaths. We have not seen that yet," he said, noting that Israel's operations were mostly in a corridor on the outskirts of Rafah.

Asked if he was saying the recent ground operations in Rafah would not prompt a US withdrawal of more military aid, Kirby said "I believe that's what I've been saying here".

The Israelis have said they used 37-pound bombs, precision-guided munitions ... If it is in fact what they used, it is certainly indicative of an effort to be discreet and targeted and precise. Now, obviously this had tragic results, and obviously that needs to be investigated.

John Kirby, US National Security Council spokesman 

Recent deaths in Rafah have tested President Joe Biden's promise to withhold weapons from Israel if the US ally made a major invasion of Rafah that put displaced persons there at risk.

Speaking at a ceremonial event in Washington, US Vice-President Kamala Harris said, "The word tragic doesn't even begin to describe" an Israeli airstrike on Sunday that triggered a fire in a tent camp in the Gaza city of Rafah, killing 45 Palestinians.

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Harris's remark, in response to a reporter's question, also followed what Gaza health authorities described as Israeli tank shelling of a tent camp in an evacuation area west of Rafah that killed at least 21 people on Tuesday.

Israel said that "something unfortunately went tragically wrong" in Sunday's airstrike while its military denied shelling the tent camp on Tuesday. Israel said it had targeted two senior Hamas operatives in Sunday's operation and had not intended to cause civilian casualties.

Hamas issued a statement celebrating the martyrdom of two fighters in the strike on Sunday, Kirby said, an indication that Israel was trying to go after Hamas in a "targeted, precise way".

"The Israelis have said they used 37-pound bombs, precision-guided munitions," Kirby said. "If it is in fact what they used, it is certainly indicative of an effort to be discreet and targeted and precise. Now, obviously this had tragic results, and obviously that needs to be investigated.”

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Asked whether Israel's strikes could put Biden in a difficult position, Kirby told reporters Tuesday that instead there was a real danger that Israel could become further isolated from the international community with the manner in which it is conducting operations. "So this is of concern, clearly, because it's not in Israel's best interest," Kirby said. "And it's not in our best interest for Israel to become increasingly isolated on the world stage."

The US administration's response was criticized earlier Tuesday by human rights and Arab American groups.

State Department spokesperson Matthew Miller told reporters that Washington will be closely watching Israel's probe but Israel's military operations so far in Rafah have not been as large-scale as those in central or northern Gaza

"Sadly, because of President Biden's insistence on sending more bombs to enable Netanyahu's war crimes in Rafah, this is now as much an American genocide as it is an Israeli genocide," said Nihad Awad, executive director at the Council on American-Islamic Relations.

Israeli and US officials have denounced the use of the term genocide to describe events on the ground in Gaza.

The State Department said on Tuesday that as soon as it saw reports of Sunday's Rafah incident, Washington expressed deep concern to Israel and urged an investigation, which Israel has promised.

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State Department spokesperson Matthew Miller told reporters that Washington will be closely watching Israel's probe but Israel's military operations so far in Rafah have not been as large-scale as those in central or northern Gaza.

Global leaders have expressed horror at the fire in a designated "humanitarian zone" of Rafah where families uprooted by fighting elsewhere had sought shelter.

More than 36,000 Palestinians have been killed in Israel's offensive, Gaza's health ministry says. Israel launched its air and ground war after Hamas-led militants attacked southern Israeli communities on Oct 7, killing around 1,200 people and seizing more than 250 hostages, according to Israeli tallies.