WASHINGTON — Diplomacy is failing and only a credible threat of military attack could still deter Iran from reaching the threshold at which it cannot be stopped from developing nuclear weapons, Prime Minister Benjamin Netanyahu told the AIPAC Policy Conference in Washington Monday.

“We have to stop Iran’s nuclear enrichment program before it’s too late. Words alone will not stop Iran. Sanctions alone will not stop Iran. Sanctions must be coupled with a clear and credible military threat if sanctions fail,” Netanyahu told a room of some 13,000 AIPAC supporters by satellite feed from Jerusalem.

“Time after time the world’s leading powers have tabled offers,” Netanyahu said. But “diplomacy has not worked. Iran ignores all these offers. It’s running out the clock.”

Iran has faced a growing number of increasingly painful sanctions that have severely hurt the Iranian economy, but even sanctions “have not stopped the nuclear program. Sanctions have hit Iran’s economy hard, that’s true, but Iran’s leaders grit their teeth” and continue to expand the nuclear enrichment program, he said.

“To prevent Iran from getting nuclear weapons, we cannot allow Iran to cross that red line,” Netanyahu emphasized in a reference to a red line he drew at his United Nations General Assembly speech in September when he argued that if Iran’s nuclear development reached a certain point, it could no longer be prevented from developing nuclear weapons.

In an earlier speech, US Vice President Biden told the crowd that President Barack Obama isn’t bluffing when he says he’ll use military action, if ultimately necessary, to prevent Iran from acquiring a nuclear weapon.

Biden said the US still prefers a diplomatic option, but that the window for that is closing.

He added that every other option must be exhausted to ensure the world community will be supportive if there’s a need for a military intervention.

Efforts to delegitimize Israel as a Jewish state are the most dangerous change he’s seen related to Israel’s security, he told the conference.

Obama’s March 20 visit to the Jewish state will deal with Iran, the Syrian civil war, and ongoing efforts to restart peace talks with the Palestinians, Netanyahu said.

“Israel is prepared for a meaningful compromise, but as Israel’s prime minister I will never compromise on our security,” Netanyahu said, noting that Israel had withdrawn from south Lebanon and Gaza, “and we got terror. That can’t happen a third time.”

“Israel seeks a peace with our Palestinian neighbors, a peace that will end our conflict once and for all,” he insisted, “but that peace must be grounded in reality and in security…. In the Middle East, a peace you cannot defend will not hold for five minutes.”

Israel must be able to defend itself by itself, he said. “The rebirth of Israel is one of the greatest events in history,” Netanyahu continued. “We shall always defend the one and only Jewish state.”

Netanyahu also noted that it must be ensured that the country’s neighbors are telling their children to live in peace.

“With President Obama we shall work for peace,” he said.

Netanyahu’s caution was in keeping with Biden’s own decidedly pessimistic tone on the prospects of Israeli-Palestinian peace talks.

“It’s going to require hard steps on all sides, but we all have a profound interest in peace,” Biden told the conference. “We’ve got to get caught trying,” he quipped, quoting former US president Bill Clinton.

“So we remain deeply engaged. As President Obama has said, while there are those who question whether this goal will ever be reached, we have no apologies for continuing to pursue this goal.”

In his comments, Netanyahu also hinted at continued Israeli operations to prevent Syria’s stockpiles of nonconventional weapons from reaching terror groups such as Hamas and Hezbollah as the country disintegrates further into civil war.

“Terror groups like Hezbollah are trying to seize these weapons as we speak, like a pack of hyenas feeding off a carcass, and the carcass isn’t even dead yet. Here, too, President Obama fully appreciates Israel’s need to defend itself.”

He noted that the civil war to Israel’s north could become “a strategic crisis.” Chemical and anti-aircraft weapons and other sophisticated arms could fall into the hands of terrorist organizations that could hit Israel or the US, he noted.

The Associated Press contributed to this report.