Iran's Islamic Revolutionary Guard Corps says Hamas political leader Ismail Haniyeh was killed by a short-range projectile fired from outside the complex where he was staying.

Haniyeh was killed on Wednesday while visiting the Iranian capital, Tehran.

The Guard issued a statement on Saturday saying that the action was planned and carried out by Israel, with support from the United States.

The statement calls the killing an act of terrorism. The Guard claims the projectile carried a 7-kilogram warhead and had been fired from outside the premises where Haniyeh was staying.

The elite paramilitary organization vowed again to retaliate against Israel, saying it will be harshly punished "in the suitable time, place and capacity."

US media outlets earlier reported that Haniyeh was killed by a bomb planted in his room.

A head of the Iranian parliament's national security committee told Arab media that retaliation against Israel is inevitable.

He said it would give the enemy an unforgettable lesson and serve as a deterrent.

Meanwhile, Israeli forces stepped up their offensives across the Gaza Strip.

Local media reports say at least 15 people were killed on Saturday in an attack on a school in northern Gaza. Women and children were reportedly among the victims taking shelter there.

Health officials in Gaza say the death toll has reached 39,550 since the conflict began last October.