Questions are being asked on how many deaths it would take to make the United States change its policy of backing Israel amid increasing outcry over the traumatic civilian deaths in Israel's bombing of the Rafah "safe zone".

Recent deaths in Rafah have tested US President Joe Biden's promise to withhold weapons from Israel if a major assault on Rafah would put displaced people there at risk.

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 did not constitute a major ground operation that crosses any US red lines.

"The Israelis have said this is a tragic mistake," US 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 does not have "a measuring stick here or a quota", Kirby said.

When asked whether Israel's strikes could put Biden in a difficult position, Kirby told reporters that there was a real danger instead that Israel could become further isolated from the international community with the manner in which it is conducting operations.

Israel, ignoring international voices and disregarding the order of the International Court of Justice, has rolled more tanks into the heart of Rafah and prepared more ground troops after the United Nations Security Council emergency meeting on Tuesday.

Speaking at a ceremonial event in Washington, US Vice-President Kamala Harris said "the word tragic doesn't even begin to describe" the Israeli strike that killed 45 people.

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

In Beijing, Chinese Foreign Ministry spokeswoman Mao Ning said on Wednesday that China is gravely concerned over Israel's military operation in Rafah.

Concerns expressed

"We strongly call on Israel to listen to the overwhelming appeal from the international community and stop pushing ahead its offensive in Rafah," Mao said at a regular news briefing.

Noting that the latest round of the Palestine-Israel conflict has made the humanitarian situation extremely dire in Gaza, Mao said China calls on relevant parties to make the cease-fire a reality at once and do everything possible to prevent casualties of innocent civilians, so as to prevent an even worse humanitarian catastrophe in Gaza.

Mao also said China is always committed to promoting talks for peace and de-escalation.

"We hope relevant countries can work together for this end," she said.

Palestinian rights advocates argue that the Biden administration is redefining what it considers to be an invasion of Rafah so as to be able to deny that an offensive is happening, Al Jazeera reported.

Yasmine Taeb, legislative and political director for advocacy group MPower Change Action Fund, called Biden's "red line" on Rafah "absolutely meaningless and merely a continuation of his callous and indefensible Gaza policy".

More than 36,171 Palestinians were killed in Israel's offensive in the Gaza Strip since Hamas' surprise attack on Oct 7, which caused about 1,200 Israeli deaths, data from Palestinian and Israeli sources showed. More than 1 million Palestinians are displaced, starved and threatened by bombs.

Agencies and Mo Jingxi in Beijing contributed to this story.