Mourners pray next to dead bodies at Abu Yousef al-Najjar hospital, in Rafah, Gaza Strip, on Saturday. Reuters

Gaza's Hamas rulers warned on Saturday that Israel's planned army operation in overcrowded Rafah could cause "tens of thousands" of casualties in the city, the last refuge for displaced Palestinians.

Israeli Prime Minister Benjamin Netanyahu has ordered the army to set its sights on Rafah. He told military and security officials late Friday to "submit to the cabinet a combined plan for evacuating the population and destroying the battalions" of Hamas in the southern city.

Hamas said in a statement that any military action would have catastrophic repercussion that "may lead to tens of thousands of martyrs and injured if Rafah... is invaded".

Netanyahu's announcement, coming after US President Joe Biden had issued his strongest criticism of Israel's response to the October 7 attack, sparked concern among world leaders and the United Nations.

"The Israeli occupation's move threatens security and peace in the region and the world. This is a blatant violation of all red lines," said the office of Palestinian president Mahmud Abbas.

Palestinians mourn relatives killed in the Israeli bombardment in Rafah, Gaza Strip, on Saturday. AP

The EU's foreign policy chief Josep Borell posted on X, formerly Twitter, Saturday: "I echo the warning by several EU member states that an Israeli offensive on Rafah would lead to an unspeakable humanitarian catastrophe and grave tensions with Egypt.

"Resuming negotiations to free hostages and suspend hostilities is the only way to avert a bloodshed."

Jordan's Foreign Minister Ayman Safadi warned "another bloodbath in Gaza cannot be allowed", in a post on X.

British Foreign Secretary David Cameron said he was "deeply concerned about the prospect of a military offensive in Rafah.

"The priority must be an immediate pause in the fighting to get aid in and hostages out," he posted.

Biden frustration

The war in Gaza was sparked by Palestinian Islamist group Hamas's unprecedented October 7 attack on Israel, which resulted in the deaths of about 1,160 people, mostly civilians, according to an AFP tally based on official figures.

Vowing to eliminate Hamas, Israel launched a massive military offensive in Gaza that the territory's health ministry says has killed at least 28,064 people, mostly women and children.

The United States is Israel's main international backer, providing it with billions of dollars in military aid.

A Palestinian man carries the body of a child killed in Israeli bombardment in Rafah, Gaza Strip, on Saturday. AFP

The US State Department has said it does not support a ground offensive in Rafah, warning that, if not properly planned, such an operation risks "disaster".

In a sign of growing frustration, Biden issued his strongest criticism of Israel yet on Thursday, describing the retaliation for Hamas's October 7 attack as "over the top".

Biden said there are "a lot of innocent people who are starving... in trouble and dying, and it's got to stop."

But Netanyahu's office said it would be "impossible" to achieve the war's objective of eliminating Hamas while leaving four of its battalions in Rafah.

Agence France-Presse