Vice Prime Minister Shaul Mofaz also criticised Russia for arming Damascus and repeated Israel's demand for international military intervention to topple Assad, akin to last year's campaign in Libya.
Israel has until recently been slow to call for Assad's fall, wary of worsening the turmoil in Syria - the two countries are enemies but have been in a mostly stable stand-off for decades.
With hourly media reports in Israel of Syrian civilian deaths, public anger has been growing and Israeli officials have been stepping up their criticism.
"A crime against humanity, genocide, is being conducted in Syria today. And the silence of the world powers is contrary to all human logic," said Mofaz during an interview on Israel's Army Radio.
Foreign powers were "making do with flaccid condemnation" rather than intervening to overthrow Assad, he added.
"Worse than that is the Russian conduct, which weakly condemns the slaughter while continuing to arm Assad's murderous regime. Best-case, this is irresponsibility, and worst-case, it is a partnership in the slaughter," Mofaz said.
A longtime Syrian ally, Russia opposes outside intervention against Damascus. Moscow has denied supporting any side in the conflict or providing arms that could be used in a Syrian civil war.
Russia says it would be open to Assad's exit from power as long as it was a result of an inclusive political process among Syrians.
Mofaz, a former top general and political centrist who joined Prime Minister Benjamin Netanyahu's conservative coalition government last month, said Israel had limited options on Syria but had to lobby for international action.
"We need to enlist the West. We need our voice to be heard. This slaughter is being carried out not far from Israel's border," he said.
"We cannot get involved, for understandable reasons. But I think that the West, led by the United States, has an interest in guarding the threshold (so) genocide does not take place."
Such language is especially loaded in Israel, which was founded in part as a haven for survivors of the Holocaust.
Speaking separately on Israel Radio, Deputy Foreign Minister Danny Ayalon said the Netanyahu government was prepared to help Syrians who take refuge in Jordan and other countries with ties to the Jewish state.