Thousands have turned out for the funeral procession in Iran of the Hamas political chief, Ismail Haniyeh, who was killed in a strike in Tehran on Wednesday.
Iran's Supreme Leader, Ayatollah Ali Khamenei, led prayers for the Hamas leader, whose body will be buried in Qatar.
US media have quoted Iranian officials as saying the supreme leader has ordered a direct attack against Israel, which it says was responsible for the attack on Iranian soil.
Israel has not commented on the assassination directly.
However, Israel's Prime Minister Benjamin Netanyahu said his country had delivered “crushing blows” to its enemies in recent days, including the killing of a senior Hezbollah commander in Lebanon hours before the Tehran strike.
He warned Israelis that “challenging days lie ahead”, as fears of a wider conflict in the Middle East grow.
"Since the strike in Beirut, we have heard threats from all sides," he told a televised address.
"We are prepared for any scenario and we will stand united and determined."
UN Secretary-General Antonio Guterres warned against a "dangerous escalation" of hostilities in the region.
Hamas's armed wing said the death of Haniyeh, who was widely viewed as the group's overall leader, would "take the battle to new dimensions" and have major repercussions.
While Haniyeh's funeral was still taking place, Israel's military said it had confirmed that Hamas's military chief, Mohammed Deif, was killed in an air strike in the Gaza Strip last month.
Deif was targeted in the strike on a compound in the Khan Younis area on 13 July. Hamas is yet to confirm his death.
Israel says Deif was one of the figures responsible for planning the 7 October attacks in southern Israel in which 1,200 people were killed. Since the attack, Israel has vowed to destroy Hamas.
Haniyeh, who played an important role in negotiations over a ceasefire in Gaza, was killed hours after attending the inauguration of Iran's new president, Masoud Pezeshkian, in Tehran.
A senior Hamas official told the OceanNewsUK the killing took place in the same building where Haniyeh had stayed during previous visits to Iran.
Three Hamas leaders and a number of guards were with him in the same building, they said.
Khalil Al-Hayya, a senior Hamas official, told a news conference that a missile hit Haniyeh "directly", citing witnesses who were with him.
The group's leadership has been left in "a state of shock", top Hamas officials have told the OceanNewsUK.
Hours before Haniyeh was targeted, Israel said it had killed senior Hezbollah leader Fuad Shukr in Beirut.
Israel believes he was responsible for a rocket attack that killed 12 people in the Israeli-occupied Golan Heights on Saturday. Hezbollah has denied any involvement.
The Iran-backed group confirmed Shukr's body was found among the rubble of the residential building which was struck. Four other people, including two children, were killed.
