Israeli Prime Minister Benjamin Netanyahu said Thursday that a firm ultimatum to Iran is the only peaceful way to stop the regime in Tehran from getting atomic weapons, increasing pressure on President Obama weeks before the U.S. presidential election.

Netanyahu’s address to the U.N. General Assembly was a highly public argument for a stronger U.S. threat to attack Iran if it does not back off from what the Israeli leader described as the final push toward a nuclear weapon. Israel and the United States say the program is intended to develop a weapon, an accusation that Iran denies.

Netanyahu made a case, laced with historical references, for telling Iran explicitly where it must stop to forestall an outside attack. He also warned that time was running out.

“At this late hour there is only one way to peacefully prevent Iran from getting atomic bombs,” Netan­yahu told the annual gathering. “And that is by placing a clear red line on Iran’s nuclear weapons program.”

Estimates have varied widely on when Iran might have a nuclear weapon, but Netanyahu offered Israel’s most specific timetable yet when he said Tehran’s progress would be irreversible by next spring or summer.

In a bit of theater, Netanyahu illustrated his point by holding up a placard showing a cartoon-like bomb with a lighted fuse. Lines on the chart marked what he said was Iran’s progress toward a weapon. With a flourish, Netanyahu pulled a red marker from his pocket and drew a thick line across the cartoon just below the start of what he described as the third and final stage.

Some of the diplomats in the audience squinted to see the chart; others seemed perplexed. The image quickly went viral on the Internet and drew praise in some quarters and criticism from opposition politicians back in Israel.

Netanyahu never directly threatened his own attack on Iran, and his tone toward Obama was conciliatory, but his meaning was clear: If Iran won’t back down and the United States won’t act, Israel will be forced to do so.

The Obama administration has been irritated by what many advisers see as rising Israeli threats and pressure. Obama told the U.N. session on Tuesday that there is still time to negotiate a peaceful end to the most troublesome elements of Iran’s nuclear program. But he has refused to set a deadline for Iran to back down or to publicly outline precisely what Iranian nuclear milestone would trigger a U.S. attack.

U.S. officials have said that an ultimatum now could kill chances for a peaceful deal to head off the Iranian program. The threat of war or a rupture with Israel, a close U.S. ally, is also an unwelcome topic in this election season.

Republican presidential nominee Mitt Romney has criticized Obama for being too hard on Israel and not hard enough on Iran. He responded to Netanyahu’s remarks with a statement saying: “I, like the rest of the American people, applaud the bravery of the people of Israel and stand with them in these dangerous times. The designs of the Iranian regime are a threat to America, Israel, and our friends and allies around the world.”

Over the past six months, Netan­yahu has made an increasingly public case for a unilateral Israeli attack on Iran sometime soon. Some analysts think he is bluffing to force Obama’s hand, while others think he is serious.

Israeli leaders say they cannot afford to risk a nuclear weapon in the hands of a clerical regime pledged to Israel’s destruction.

The Israeli demand for a U.S. red line on Iran is the latest point of conflict in a testy personal relationship between Obama and Netan­yahu. Although U.S. officials regularly profess an unshakable alliance with Israel, the two leaders have done little to hide their mutual suspicion and dislike.

But in his speech, Netanyahu thanked the president for ruling out a Cold War-style containment strategy for Iran, saying that the clerical regime in Tehran would not be as responsible as Soviet leaders. And he talked about the United States and Israel working together to stop Iran.

The Israeli leader devoted much of his speech to portraying a nuclear-armed Iran as a threat not only to Israel but also to the entire world. He equated the threat of an Iranian regime with an atomic weapon to a nuclear-armed al-Qaeda, saying, “They’re both fired by the same hatred, they’re both driven by the same lust for violence.”

“Who would be safe in Europe? Who would be safe in America? Who would be safe anywhere?” he asked.

Iran’s mission to the United Nations issued a lengthy refutation of Netanyahu’s accusations, calling them baseless.

In Israel, Shelly Yachimovich, leader of the opposition Labor Party, scoffed at Netanyahu’s use of a simple diagram to highlight his argument for a red line.

“The drawing is very attractive,” she said on Channel 10 television, “but that is not what will influence the United States to become deeply involved and be our ally in every step we take.”

A new Israeli government report leaked to local media concludes that international sanctions are hitting Iran hard, possibly undermining Netanyahu’s argument for tougher action.

Shaul Mofaz, who heads the centrist Kadima party, said that plans to confront Iran should take place behind closed doors. Speaking on Israel’s Channel 2 television, Mofaz noted that Obama “has said that Iran will not have a bomb . . . that he will prevent Iran from going nuclear. I think that’s a clear red line.”

U.S. military and intelligence officials have said that an attack by Israel would be unlikely to destroy the Iranian program, possibly leaving the United States to finish the job and igniting a new war in the Middle East.

Joel Greenberg in Jerusalem contributed to this report.