Some 600,000 children packed into Gaza's Rafah city face "further catastrophe," UNICEF warned Monday, urging against their forced relocation after Israel ordered an evacuation ahead of its long-threatened ground invasion.

"Given the high concentration of children in Rafah... UNICEF is warning of a further catastrophe for children, with military operations resulting in very high civilian casualties and the few remaining basic services and infrastructure they need to survive being totally destroyed," the United Nations children's agency said in a statement.

It said Gaza's youth were already "on the edge of survival," with many in Rafah -- where the agency said the population has soared to 1.2 million people, half of them children -- already displaced multiple times and with nowhere else to go.

"More than 200 days of war have taken an unimaginable toll on the lives of children," said UNICEF executive director Catherine Russell.

"Rafah is now a city of children, who have nowhere safe to go in Gaza," she said, warning that a large-scale military operation by Israel would bring "chaos and panic, and at a time where (children's) physical and mental states are already weakened."

UNICEF estimates that Rafah's population has swelled to nearly five times its normal figure of 250,000 residents.

Calling again for a ceasefire and for safe access for humanitarian organizations, the agency highlighted there are some 78,000 infants under age two sheltering in the city, along with 175,000 children under five who are affected by infectious disease.

Gaza's bloodiest-ever war began following Hamas's unprecedented October 7 attack on Israel that resulted in the deaths of more than 1,170 people, mostly civilians, according to an AFP tally of Israeli official figures.

Vowing to destroy Hamas, Israel has conducted a retaliatory offensive that has killed at least 34,735 people in Gaza, according to the Hamas-run territory's health ministry.

Of that toll, more than 14,000 are children, the ministry has said.

Israeli Prime Minister Benjamin Netanyahu has vowed to send ground troops in against Hamas fighters in Rafah regardless of any truce, and despite concerns from the United States, other countries and aid groups.