US President Joe Biden said Wednesday he would stop US weapons supplies to Israel if it attacks Rafah in southern Gaza, his most direct warning to date over the prosecution of the war against Hamas.

In an interview Biden also deplored the fact that civilians had been killed by the dropping of US bombs on the Palestinian territory.

His fresh warning came after the US last week halted a shipment of huge American bombs to Israel as it appeared ready to proceed with a major attack on Rafah — a city packed with Palestinian civilians sheltering near the Egyptian border.

“If they go into Rafah, I’m not supplying the weapons that have been used… to deal with the cities,” Biden said. “We’re not gonna supply the weapons and the artillery shells that have been used.”

Biden, a self-described Zionist, had long resisted stopping any of the $3 billion in weapons the United States sends each year to Israel — and pushed Congress for an increase in the wake of the October 7 attack by Hamas that triggered the major Israeli retaliation.

But US officials say privately that his hand was forced after Israeli Prime Minister Benjamin Netanyahu made clear he would go ahead with an assault on Rafah, defying Biden’s public appeals to spare the city.

Under increasing pressure from the left of his own party to limit arms shipments, the Biden administration paused delivery last week of 1,800 2,000-pound (907 kilogram) bombs and 1,700 500-pound bombs.

“Civilians have been killed in Gaza as a consequence of those bombs,” Biden said. “It’s just wrong.”

“We’re not walking away from Israel’s security,” Biden insisted in the interview. “We’re walking away from Israel’s ability to wage war in those areas.”

The Biden administration has previously taken smaller steps to show displeasure with Netanyahu, including imposing sanctions on extremist Israeli settlers and letting through a UN Security Council resolution that supported a ceasefire.

Major US interventions in the past have changed Israeli behavior. In 1991, Israel begrudgingly attended the Madrid conference that led to a peace process with the Palestinians after then-president George W. Bush held up US loan guarantees to build settlements.

In 1956, heavy US pressure including economic threats forced Israel as well as Britain and France to give up their grab of the Suez Canal from Egypt.