Iranian Revolutionary Guards' Quds Force chief Ismail Qaani says in a letter to the new leader of Hamas that Tehran will avenge the killing of his predecessor who was killed in the Iranian capital last week.

The letter, of which a copy was seen by The Associated Press, came days after the leadership of Hamas chose Yahya Sinwar as its new leader replacing the late Ismail Haniyeh, who was killed during a visit to Iran.

Since Haniyeh was killed in an explosion during a visit to Iran, tension has been rising in the region as Tehran blamed Israel for his death and vowed to retaliate.

“We are preparing to avenge his blood, a painful and difficult incident that happened in the Islamic Republic of Iran. This is our duty,” Qaani told Sinwar about Haniyeh. He didn't elaborate on how Teheran will avenge Haniyeh’s death.

Qaani said that Haniyeh’s blood will “make the harsh punishment to the Zionist entity at the hand of the Islamic Republic” harsher that previous ones.

He was apparently referring to the mid-April missile and drone attack that Iran launched against Israel to avenge an Israeli airstrike on the Iranian Consulate in the Syrian capital in which two Iranian generals were killed.

Qaani vowed in the letter to Sinwar that Tehran “will be with you on the road of resistance until we achieve the divine promise which is to clear Jerusalem."