JUST IN: Hamas political leader Ismail Haniyeh was assassinated in Tehran using an explosive device that had been covertly hidden in the guest house where he was staying.

Haifa, Israel (CNN) – Hamas political leader Ismail Haniyeh was assassinated in Tehran on Wednesday using an explosive device that had been covertly hidden in the guest house where he was staying, a source familiar with the matter told CNN.

According to the source, who had been briefed on the operation, the bomb was concealed about two months ago in the guest house where Haniyeh was known to stay in Tehran and detonated remotely once he was inside his room there.

The Iranian government and Hamas say Israel carried out the assassination. Israel has neither confirmed nor denied its involvement.

US officials were briefed on the operation by Israeli officials only after the assassination, the source said.

The New York Times was the first to report the details of Haniyeh’s assassination.

Iranian state media and Hamas previously indicated Haniyeh was killed by a rocket fired from outside the building.

But the revelation that a bomb was smuggled inside the guest house, which was under the protection of Iran’s Islamic Revolutionary Guard Corps, indicates a startling breach of security for the IRGC.

Iran’s Supreme Leader Ayatollah Ali Khamenei said Wednesday: “You killed our dear guest in our house and now have paved the way for your harsh punishment.”

CNN has sought comment from the Iranian government.

This is a developing story. It will be updated