Iran vows revenge after Hamas leader killed in Tehran
Israel has not commented directly on the strike which killed Ismail Haniyeh in Tehran early on Wednesday.
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 there are threats surrounding from all directions," he told a televised address.
"We are prepared for any scenario and we will stand united and determined against any threat."
UN Secretary-General Antonio Guterres warned against a "dangerous escalation" of hostilities in the region. The UN Security Council was due to meet on Wednesday evening to discuss the situation.
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.
The group attacked Israel on 7 October, killing around 1,200 people. 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 BBC 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 BBC.