WASHINGTON (AP) — As it targets Hamas’ underground tunnels in Gaza, Israel has relied on powerful 2,000-pound bombs provided by the United States. But now those deliveries are on hold.

The U.S. is pausing a shipment of 1,800 of the bombs, as well as 1,700 500-pound bombs, U.S. officials said. The decision comes as Israel is planning an assault on the southern Gaza city of Rafah in an attempt to root out the final elements of Hamas.

With more than 1 million refugees sheltering in Rafah, U.S. officials are concerned the bombs could inflict massive casualties. Human rights groups have long said that Israel’s use of powerful bombs has caused the indiscriminate killings of civilians.

Defense Secretary Lloyd Austin told a Senate panel Wednesday that smaller, more precise weapons are needed for a densely populated area like Rafah.

Still, he made clear the decision wasn’t final.