U.S. President Joe Biden expressed hope Iran would stand down despite its threat to avenge the assassination of Hamas' leader in Tehran, as fears mounted that Israel's war against Palestinian militants in Gaza could escalate into a wider Middle East conflict.

Regional tensions have soared following the assassination of Ismail Haniyeh, Hamas' top leader, on Wednesday, a day after an Israeli strike in Beirut killed Fuad Shukr, a senior military commander from the Lebanese group Hezbollah, which like Hamas is backed by Iran.

Iran and Hamas have blamed Israel for Haniyeh's killing, and they, together with Hezbollah, have vowed revenge. Israel has not claimed or denied responsibility.

Asked by reporters whether Iran would stand down, Biden said on Saturday in response to a shouted question, "I hope so. I don't know."

Seeking to bolster defenses in the Middle East in response to threats from Israel's foes, the Pentagon said on Friday it would deploy additional fighter jets and Navy warships to the region.

Haniyeh's death was one in a series of killings of senior Hamas figures as the Gaza war nears its 11th month, and it fuelled concern that the conflict in Gaza was turning into a wider Middle East war.

Hamas said it had begun a "broad consultation process" to choose a new leader three days after the assassination of Haniyeh, who was the face of the group's international diplomacy.

The U.S. and international partners including France, Britain, Italy and Egypt continued diplomatic contacts on Saturday seeking to prevent further regional escalation.

The U.S. urged its citizens who wish to leave Lebanon to start making plans immediately, and the British government advised its nationals to "leave now." Canada warned citizens to avoid all travel to Israel, saying the regional armed conflict endangered security.

Violence continued on Saturday in the Palestinian territories.

An Israeli airstrike on a school sheltering displaced persons in Gaza City killed at least 15 Palestinians on Saturday, hours after two strikes in the occupied West Bank killed nine militants including a local Hamas commander, Hamas said.

The Israeli military said the first of two West Bank airstrikes hit a vehicle in a town near the city of Tulkarm, targeting a militant cell it said was on its way to carry out an attack.

A Hamas statement said one of those killed was a commander of its Tulkarm brigades, while its ally Islamic Jihad claimed the other four men who died in the strike as its fighters.

Hours later, a second airstrike in the area targeted another group of militants who had fired on troops, Israel's military said, during what it described as a counterterrorism operation in Tulkarm.

Palestinian news agency WAFA said four people had died in that strike, and Hamas said all nine of those killed in the two Israeli attacks in the West Bank were fighters.

At least 39,550 Palestinians have been killed in the Israeli military campaign in Gaza, according to Gaza health officials. The offensive was triggered by a Hamas-led attack on southern Israel on Oct. 7 in which 1,200 people were killed and 250 abducted, according to Israeli tallies.

A high-level Israeli delegation made a brief visit to Cairo on Saturday in an attempt to resume Gaza ceasefire negotiations, Egyptian airport authority sources said. The Israeli officials returned to Israel hours later, Israeli media said.

Chances of a breakthrough appear low following the latest incidents in Lebanon and Iran. Israel has not said whether or not it was behind Haniyeh's assassination. But Netanyahu said earlier this week that Israel had delivered crushing blows to Iran's proxies of late, including Hamas and Hezbollah.