White House national security adviser Jake Sullivan reiterated Biden administration pressure on Israel to limit its military operation in Gaza during a visit to the country on Sunday.

Sullivan is on a combined trip to Israel and Saudi Arabia as the U.S. leans on its lead Arab ally in the region to calm tempers in the Middle East amid the Israel-Hamas war. 

The White House said before Sullivan’s meeting Sunday that he would urge Israeli Prime Minister Benjamin Netanyahu to make limited operations in Rafah instead of a full-on invasion, as has happened in the rest of Gaza.

Over a million people are believed to be sheltered in Rafah, the last remaining major settlement that has not yet been invaded by Israeli ground forces. Israel has shelled the city for months, accelerating concerns of a growing humanitarian crisis.

At least 28 Palestinians were killed on Sunday, Gaza health officials and Hamas said, most of them in a strike on a house in Nuseirat in the central Gaza Strip.

Biden has put down a hard line on Rafah, promising to halt military aid to Israel if it invades the city. Despite encroaching on parts of the city, the Biden administration has said current operations have not crossed the line to trigger any arms halt.

The pressure comes as the Israeli government scrambles to find a plan for Gaza, with Hamas popping up again in regions previously cleared by the Israeli military. Israeli opposition leader Benny Gantz said Saturday that he will leave the war cabinet if Netanyahu does not create a solid plan to govern Gaza by next month.

The U.S. and Saudi Arabia are also said to be in final negotiations over a security agreement, which Sullivan discussed with Saudi Arabia Crown Prince Mohammed bin Salman over the weekend. 

Relations between Israel and Saudi Arabia have severely deteriorated since the outbreak of the Israel-Hamas war, when the two nations neared a previously-unheard-of diplomatic normalization deal.