The reclusive cleric is one of the most politically influential figures in Iraq in the wake of the U.S.-led 2003 invasion.
The Iranian Press TV reported Monday that Sistani called on leaders throughout the Middle East to take action beyond criticizing the Israeli government for what is considered aggressive force by some governments.
"Arab and Islamic nations need to take a decisive stance, now more than ever, to end these ongoing aggressions and to break the unjust siege imposed on the brave people of Gaza," the cleric said in statements from his office in Najaf.
Hamas had declared an end to a six-month cease-fire against Israeli Dec. 19, launching a salvo of rockets into the southern city of Ashkelon. Israeli forces in response launched a massive airstrike on Hamas targets in the Gaza Strip during the weekend, killing at least 300.