The chief prosecutor of the International Criminal Court (ICC) warned Israel it could face war crimes charges if it pushes ahead with an invasion of Rafah.

Karim Khan, a British lawyer, said he was “deeply concerned” about the potential assault on the southern Gazan city where 1.4 million Palestinians are understood to be sheltering, and reiterated that those who commit war crimes “will be held accountable”.

International pressure was mounting on Monday against the planned attack on the last remaining Gazan enclave, with Lord Cameron urging Israel to “stop and think seriously” before taking any further action.

The Foreign Secretary said Britain was “very concerned” after Israel ordered troops on standby to close in on the city, and called again for a “sustainable ceasefire”.

Meanwhile, Volker Türk, the UN’s human rights chief, said the prospect of a military operation where many refugees have already fled from bombs was “terrifying”.

“A potential full-fledged military incursion into Rafah, where some 1.5 million Palestinians are packed against the Egyptian border with nowhere further to flee is terrifying, given the prospect that an extremely high number of civilians, again mostly children and women, will likely be killed and injured”, he said.

His comments came after an Israeli special forces rescue operation in the early hours of Monday morning freed two Israeli-Argentine hostages held by Hamas militants in Rafah. Supporting airstrikes, used as a diversion as the hostages escaped, killed at least 67 people.

Joe Biden, who has warned against launching an operation in the region without an extensive evacuation plan, is considering publicising his frustrations with Benjamin Netanyahu.

The US president is said to have referred to the Israeli leader as an “a--hole” who is impossible to work with and sees him as the main impediment to peace, according to NBC News.

On Monday, Josep Borrell, the EU’s foreign policy chief, called on the US and allies to stop sending weapons to Israel as “too many people” were being killed.

“Well, if you believe that too many people are being killed, maybe you should provide less arms in order to prevent so many people being killed,” Mr Borrell told reporters after a meeting of EU development aid ministers in Brussels.

Mr Borrell pointed to a ruling by a Dutch appeals court on Monday blocking the export of F-35 fighter jet parts to Israel over a “clear risk of violations of international humanitarian law” in Gaza.

Aid agencies say an assault on Rafah would be catastrophic. Egypt has reinforced its border with the city, saying it fears Gazans will be pushed across, never to return.

Israel has insisted its Gaza operation is not over until it has destroyed Hamas and its leadership, many of whom are thought to be hiding in Rafah.

On Monday, the White House said there were “legitimate” targets in Rafah, including the Hamas chiefs known to have travelled south and hidden in a network of tunnels.

Eylon Levy, an Israeli government spokesman, said on Monday that Israel was fighting to prevent “extremism and terrorism from spiralling further in Europe” and said it expected its allies to stand by it.

Mr Levy called on UN aid agencies to help facilitate the offensive, urging them to “work with us to find a way”.

Mr Netanyahu has brushed aside international alarm over his plans, saying those who said Israel should not enter Rafah “are basically saying lose the war”.

On Friday, he announced he had ordered his military to prepare a plan to evacuate the city, a key corridor for humanitarian assistance in southern Gaza where Palestinians had previously been told to flee to for safety.