Israel said on Thursday it believed Syrian forces had used chemical weapons in the killing of hundreds of people in the rebel-held suburbs of Damascus, and accused the world of turning a blind eye to such attacks.
Israeli Prime Minister Benjamin Netanyahu said if Syria is not punished, its ally Iran could be encouraged to develop nuclear weapons.
"Syria has become Iran's testing ground, and Iran is closely watching whether and how the world responds to the atrocities committed by Iran's client state Syria ... against innocent civilians in Syria," Netanyahu said in a statement.
"These events prove yet again that we simply cannot allow the world's most dangerous regimes to acquire the world's most dangerous weapons."
Opposition activists have accused Assad's forces of gassing hundreds, including women and children, on Wednesday, allegations which government officials deny.
French Foreign Minister Laurent Fabius said on Thursday the international community needed to respond with force if the allegations of a Syrian government chemical attack proved true, although there was no question of sending troops on the ground.
For Israel, the conflict in its northern neighbour is a battle between two evils: Assad - who is allied with two of its most strident enemies, Iran and Lebanon's Hezbollah guerrillas - and Sunni jihadists fighting with rebels to oust him.
Wednesday's incident, carried out while U.N. inspectors were in Damascus to look into allegations of earlier chemical attacks by Syrian President Bashar al-Assad's forces, was seized by Israel as an opportunity to question international resolve to curb its foes' suspected pursuit of weapons of mass destruction.
"It is absurd that the UN investigators, who are right now in Damascus to verify use of chemical weapons, are prevented from reaching the afflicted areas by the Syrian regime," Netanyahu said.
The Assad government has denied using chemical weapons against Syrians. Iran denies seeking nuclear weapons. Israel is widely assumed to have the Middle East's only nuclear arsenal.
Interviewed on Israel Radio, Strategic Affairs Minister Yuval Steinitz said that according to "Israeli intelligence assessments", chemical weapons had been used in the rebel-held eastern Damascus suburbs, and "not for the first time" in Syria's civil war.
Steinitz did not provide further details. Israeli Defence Minister Moshe Yaalon made similar remarks on Wednesday.
"Nothing tangible or significant has been done in the past two years to halt Assad's incessant massacre of his citizens," Steinitz said. "The world condemns, the world investigates, the world pays lip service."
Israel has stopped short of urging Western military intervention in the Syrian conflict.
Israel has on several occasions taken action of its own, firing into Syria after mortar bombs and shells from battles near the frontier struck inside the Israeli-occupied Golan Heights. Israel captured the Golan from Syria in a 1967 war.
Yisrael Katz, Israel's transportation minister, said the alleged horror of gas attacks on Syrians resonated strongly in the Jewish state, founded after the Nazi Holocaust in which many of the six million Jewish dead were killed in gas chambers.
Israel has long conducted a national gas mask distribution programme for the civilian population. It has accused Syria of stockpiling chemical weapons and voiced concern they could be transferred to Hezbollah or other hostile groups.
"Today he (Assad) is murdering his own people, tomorrow he will threaten us and perhaps worse," Katz told Israel Radio.
Additional reporting by Ori Lewis; Editing by Pravin Char and Sonya Hepinstall