JERUSALEM — After an inconclusive round of talks this month between world powers and Iran about its nuclear program, Israeli leaders are expressing deep skepticism about the chances for a diplomatic breakthrough and urging tougher international demands.

Following months of advocating stricter sanctions against Iran, Israeli officials are now pushing for a halt to all its uranium enrichment activity, warning that further delays could put the Iranian nuclear program beyond the reach of an effective military strike.

The Israeli statements, ahead of another planned round of talks in Moscow in June, reflect concerns that the sense of urgency that pervaded earlier discussions on Iran has faded in recent months and that Washington and other nations are allowing the Iranian leadership to gain more time to pursue what Israel says is a drive toward nuclear weapons.

“We have no illusions about the talks,” Defense Minister Ehud Barak said Wednesday at the annual conference of the Institute for National Security Studies at Tel Aviv University, a research center with close links to Israel’s military and security establishment.

Calling for stronger international action against Syrian President Bashar al-Assad after the massacre of more than 100 people in the town of Houla last week, Barak warned that the obstacles to mobilizing intervention to stop the bloodshed in Syria were a lesson regarding Iran.

“The difficulty the international community is having in generating the ability to act even in such a clear-cut case . . . must tell us something about other areas as well,” Barak said.

It is far from certain that “if only conditions are created in which it is self-evident and clear that the world must act, it will indeed act,” Barak added. “It is not self-evident, and it is not clear.”

Israel and the United States have different timetables for assessing the Iranian nuclear effort, Barak said, asserting, “Our clock is ticking faster.” He repeated earlier warnings that Iran is approaching a “zone of immunity” in which its nuclear facilities would be dispersed and protected underground, beyond the reach of a military strike.

“The relevant moment diplomatically is the last moment you can do something about the issue,” Barak said, adding that “the Iranians are systematically moving to a point after which Israel will not be able to do anything about it.”

In an address Tuesday in the same forum, Prime Minister Benjamin Netanyahu laid out what he said should be the international demands on Iran, and he criticized what he called a softening of the stance of Western nations in the discussions.

“Iran must stop all enrichment of nuclear material, it must remove from its territory all the material that has been enriched so far, and it must dismantle the underground nuclear facility at Qom,” Netanyahu said, referring to the city near the fortified plant at Fordow. “Only an explicit Iranian commitment to implement these three demands, and explicit verification of their implementation, can stop the Iranian nuclear program.”

Netanyahu said world powers should have demanded a halt to all Iranian enrichment activities. “But instead of that, the demands are being lowered,” and low-grade enrichment, which Iran maintains is for peaceful purposes, is now being tolerated, he said.

Vice Prime Minister Moshe Yaalon, in an interview this week on Israeli Army Radio, said he detected a lack of urgency and even foot-dragging among nations negotiating with the Iranians.

“There’s a tendency in the West, and we’re familiar with this from past historical events, to avoid a confrontation — to postpone the problem to next year, to the next term, to the next generation,” Yaalon said. “It is incumbent on us, based on our historical experience, to ring the alarm bells.”

But echoes of the internal debate in Israel were also heard at the Tel Aviv conference, when two former top security officials counseled against hasty military action.

Meir Dagan, a former head of the Mossad, Israel’s overseas intelligence agency, disputed arguments that Israel faces a choice of “either bombing or the bomb.”

“If we bomb, they won’t have a bomb?” Dagan asked, arguing that an Israeli attack would only accelerate the Iranian nuclear program in response and that “we will give them legitimacy to attain nuclear military capability.”

Furthermore, he said, an Israeli attack would help the Iranian leadership overcome serious domestic challenges and unite Iranians behind their government.

“That would be a mistake,” he said.

Gabi Ashkenazi, a former army chief of staff, argued that more time should be given for economic sanctions to take their toll before contemplating a strike on Iran’s nuclear facilities.

“It’s important to exhaust all other means before using, or considering using, the military option,” he said. “There’s still time for diplomacy.”

Barak, who argued that military action now would be less costly than confronting a nuclear-armed Iran in the future, seemed dismissive of the chances for diplomatic success.

“With all the preference everyone has for diplomacy, sanctions and miracles from heaven,” he said, “no option should be removed from the table.”