The warning came after Israeli Prime Minister Benjamin Netanyahu said he had ordered troops to "prepare to operate" in Rafah, the last major town in the Gaza Strip Israeli ground troops have yet to enter.
Israel's armed forces stepped up its air strikes on the city on Thursday as fears of ground fighting grew among the hundreds of thousands of civilians displaced from other parts of Gaza who are now sheltering in tents and bombed out buildings.
United Nations chief Antonio Guterres warned that a military push into Rafah "would exponentially increase what is already a humanitarian nightmare."
Heavy fighting raged on despite international efforts toward a cease-fire in the bloodiest ever Gaza war sparked by Hamas's October 7 attack on southern Israel.
U.S. State Department deputy spokesman Vedant Patel said Washington had "yet to see any evidence of serious planning" for a Rafah ground operation, noting that Rafah is also a crucial entry point for humanitarian aid destined for Gaza, adding such an assault was "not something we'd support."
"To conduct such an operation right now with no planning and little thought... would be a disaster."
US Secretary of State Antony Blinken speaks during a meeting with former Israeli army chief of staff Gadi Eisenkot and former Israeli Defense Minister Benny Gantz, both members of the current Israeli five-person war cabinet in Tel Aviv on February 8, 2024.
Secretary of State Antony Blinken conveyed Washington's concerns to Netanyahu directly during their talks on Wednesday in Jerusalem, Patel said.
Publicly, the U.S. top diplomat warned that any "military operation that Israel undertakes needs to put civilians first and foremost."
Blinken left Israel without securing a pause in fighting, wrapping up his fifth crisis tour of the Middle East since the war started.
AFP journalists reported that Israel carried out at least seven air strikes overnight in the Rafah area, terrifying civilians crowded into shelters and makeshift camps.
"These strikes are proof there is no safety in Rafah," said resident Umm Hassan, 48, whose home was damaged in the shelling of the nearby house of a local police chief.
"Look at the residential unit they just blew up," he said. "Regarding Netanyahu's threat to invade Rafah, we are people of faith. We are not worried. Life is one and God is one."
Strikes and ground combat continued across the Hamas-ruled territory, now in its fifth month of war, where the health ministry said another 130 people were killed in 24 hours.