U.S. Secretary of State Antony Blinken appealed for “all parties” in the Middle East to avoid escalatory actions that could plunge the region into further conflict, and said Thursday that a cease-fire between Israel and Hamas was the only way to begin to break the cycle of violence and suffering.

The remarks came as prayers were held by Iran’s Supreme Leader Ayatollah Ali Khamenei and representatives of Palestinian militia groups for Hamas leader Ismail Haniyeh and his bodyguard. Worry has spread that the shock assassination risks escalating the fighting into an all-out regional war.

No one immediately claimed responsibility for Wednesday's attack, but suspicion quickly fell on Israel, which has vowed to kill Haniyeh and other Hamas leaders over the group’s Oct. 7 attack on Israel that sparked the war.

The assassination of Haniyeh in Tehran and the strike against senior Hezbollah commander Fouad Shukur in Beirut could upend the attempts to defuse a Middle East powder keg, with Iran also threatening to respond after the attack on its territory. And the Israeli military said Thursday it confirmed that the head of Hamas’ military wing, Mohammed Deif, was killed in an airstrike in Gaza in July.

During a late-July visit to the U.S., Israeli Prime Minister Benjamin Netanyahu said his country was determined to win nothing less than “total victory” against Hamas. Asked directly by journalists on the point later, he said that Israel hoped for a cease-fire soon and was working for one.