Israel’s prime minister declared Tuesday that his country will never allow Iran to get nuclear weapons, even if it has to act alone, and dismissed Iranian President Hassan Rouhani’s “charm offensive” as a ruse to get relief from sanctions.

Benjamin Netanyahu, speaking to world leaders at the U.N. General Assembly, played the spoiler to Iran’s overtures to warm ties with the U.S. after decades of estrangement. Last week, the U.S. and Iranian presidents spoke on the phone, the highest level contacts between their countries in 34 years.

Netanyahu said Israel’s future is threatened by a “nuclear-armed” Iran seeking its destruction and urged the international community to keep up pressure through sanctions.

“Israel will not allow Iran to get nuclear weapons,” he said. “If Israel is forced to stand alone, Israel will stand alone, but in standing alone Israel will know that we will be defending many, many others,” Netanyahu added.

Iran’s U.N. Ambassador Mohammad Khazaee shot back: “Unlike Israel, Iran would not and did not attack any country.”

Exercising Iran’s right of reply in the assembly, he added: “It is not due to its inability, but due to its principled policy in rejecting any use of force. … Therefore the Israeli prime minister had better not even think about attacking Iran let alone planning for that.”

At the White House, press secretary Jay Carney said Netanyahu’s skepticism about Iran and its intentions is “entirely justifiable” because until recently Iran’s leadership “was pledging to annihilate Israel.” He said the U.S. share’s Israel’s goal of keeping Iran from obtaining a nuclear weapon.

Carney stressed that Obama will be “very firm” on demanding verifiable, transparent action to ensure that Iran has given up its nuclear weapons ambitions.

Netanyahu said a nuclear-armed Iran would have a choke-hold on the world’s main energy supplies.

“It would trigger nuclear proliferation throughout the Middle East, turning the most unstable part of the planet into a nuclear tinderbox. And for the first time in history, it would make the specter of nuclear terrorism a clear and present danger,” the Israeli leader said.

Netanyahu said the greater the pressure, the greater the chance for diplomacy to succeed. He said the only diplomatic solution that would work is one that requires Iran to completely dismantle its nuclear weapons program and prevents it from starting one in the future.

This would require a halt to all uranium enrichment, removing uranium stockpiles from Iran, dismantling the infrastructure for “nuclear breakout capability” — reaching the point where the country can make a quick dash to a nuclear weapon.

He also said it would require stopping all work at a heavy water reactor aimed at producing plutonium, which like uranium can be used to produce nuclear weapons, he said.

Netanyahu called Rouhani “a loyal servant of the regime” and stressed that he has done nothing to stop Iran’s nuclear program since his election in June.

Rouhani was at the U.N. last week and presented a more moderate face of the hard-line clerical regime in Tehran.

He agreed to the first nuclear talks with six world powers since April, held on Thursday on the sidelines of the General Assembly. U.S. and European diplomats emerged from the talks saying they saw a marked shift in Iran’s tone for the better. But they also insisted it must be backed up by concrete actions to assure the world Tehran is not seeking to develop a nuclear weapon.

The parties agreed to meet again in Geneva on Oct. 15-16 for more substantive negotiations.