Israeli Prime Minister Benjamin Netanyahu said Monday that diplomacy has so far failed to deter Iran from pursuing its nuclear program, warning it was getting closer to crossing a crucial "red line."

"Iran enriches more and more uranium, it installs faster and faster centrifuges," and it is "running out the clock" on diplomatic efforts to prevent the Islamic republic from obtaining nuclear capability, Netanyahu said.

"We have to stop its nuclear enrichment program before it's too late," he warned in a speech via satellite from his office in Israel to the American Israel Public Affairs Committee, the largest pro-Israel lobby in the US.

The Israeli prime minister warned that leaders in Tehran had opted to "just grit their teeth" through punishing international sanctions and pursue their nuclear plan, come what may.

"It's still not crossed the red line I drew with the United Nations last September," Netanyahu said, referring to the point at which Israel believes Iran would be able to build a nuclear weapon.

"But Iran is getting closer to that red line, and it is putting itself in a position to cross that line very quickly once it decides to do so."

"And I have to tell you," Netanyahu added, "from the bottom of my heart, and the clarity of my brain, words alone will not stop Iran."

"Diplomacy has not worked," he said, stressing that Iran's leaders have "used negotiations, including the most recent ones, to buy time to press ahead with its nuclear program."

Sanctions alone will not stop Iran, Netanyahu said. They must be "coupled with a clear and credible military threat if diplomacy and sanctions fail."

Netanyahu will host US President Barack Obama on an official visit later this month, and he said the two leaders would discuss Iran as well as Syria, where a civil war is raging against strongman Bashar al-Assad's regime.

Netanyahu won re-election earlier this year but has so far struggled to scrape together a coalition. But he told the AIPAC audience he would soon put together a workable government.

"I intend to form a strong and stable government in the days ahead," he said. (AFP)