U.S. President Joe Biden warned Israel against an invasion of the city of Rafah in southern Gaza as cease-fire talks between Israel and Hamas remained deadlocked.

The U.S. had been hoping for a breakthrough in the negotiations before Islam’s holy month of Ramadan, which began after sundown Sunday. The deal as envisioned would see a six-week pause in fighting and the release of dozens of Israeli hostages in return for hundreds of Palestinian prisoners.

Negotiations remained at an impasse, however, with Israel accusing Hamas of stalling in a bid to inflame violence across the region during Islam’s holy month, and Hamas saying more Israeli hostages have died in captivity than earlier believed.