Joe Biden's "reputation is on the line" if Benjamin Netanyahu decides to ignore him, our Middle East correspondent Alistair Bunkall says.
He says he suspects the Israelis knew about the US leader's decision not to supply Israel with weapons for its assault on Rafah before it was made public.
"I'm sure this would have been communicated to the Israelis behind the scenes," he says.
But by making it public, the US is "setting a very clear red line".
"Therefore, Mr Biden's reputation is on the line if Benjamin Netanyahu decides to ignore him and go into Rafah regardless," he says.
"At that point Biden will have a decision to make - does he soften and side with his ally or does he stand firm to the red line he has now set?"
Mr Netanyahu may be thinking the US has set clear red lines in the past that have been ignored by leaders.
"I'm thinking the use of chemical weapons in Syria and then with Vladimir Putin and the invasion of Ukraine," Bunkall says.
"When those respective leaders have decided to ignore the US, the US has done nothing about it so perhaps Mr Netanyahu may feel this is an empty threat."