Israeli Prime Minister Benjamin Netanyahu warned Monday at the United Nations that a nuclear-capable Iran would pose the gravest threat to the world, far outstripping the jihadist terror from Iraq and Syria.

"Make no mistake, ISIS must be defeated," Netanyahu told the UN General Assembly, referring to the Islamic State group.

"But to defeat ISIS and leave Iran as a threshold nuclear state power is to win the battle and lose the war."

Netanyahu spoke a week after US President Barack Obama took to the UN podium to appeal for international support for the US-led coalition firing air strikes to defeat the IS jihadists.

The prime minister took a swipe at Iran's "smooth-talking" President Hassan Rouhani, accusing him of seeking a deal on Tehran's nuclear program without scrapping any bomb-making capacity.

"The Islamic republic is now trying to bamboozle its way to an agreement... that will cement Iran's place as a threshold military nuclear power," he said.

"Allowing that to happen would pose the gravest threat to us all."

A week-long round of talks between Iran and six world powers in New York ended on Friday with no breakthrough toward a deal to ensure Tehran's nuclear program cannot be used for military purposes.

Iran and the so-called P5+1 comprised of Britain, China, France, Russia, the United States plus Germany have set a deadline of November 24 to reach a historic agreement.

The six powers are asking Iran to scale back uranium enrichment, which could be used to make a nuclear bomb, but Tehran has long denied that its atomic program would be used to build a weapon.

- Rouhani's 'phony tears' -

The prime minister derided Rouhani, who spoke out against jihadists during his UN address last week, accusing him of shedding "phony tears" while engaging in a "terror campaign" of his own.

"To say that Iran doesn't practice terrorism is like saying Derek Jeter never played shortstop for the New York Yankees," he quipped, referring to the Yankees captain, who retired to great fanfare at the weekend.

"Would you let ISIS enrich uranium? Would you let ISIS build a heavy-water reactor?" he asked.

"Iran's nuclear military capabilities must be fully dismantled," Netanyahu told the Assembly, drawing applause.

Israel has repeatedly warned the West against making concessions to Tehran in talks on its nuclear program and asserted that it reserves the right to weigh military action to confront the Iranian threat.

Netanyahu made a splash two years ago when he turned up at the UN General Assembly with a large cartoon drawing of a bomb to illustrate the threat posed by Iran's nuclear program.

At this year's gathering, he showed a photograph purportedly of Hamas rocket-launchers with children playing next to them -- a scene the Israeli leader said proved that Hamas had used civilians as human shields in Gaza.

Netanyahu said jihadists in Iraq and Syria share a radical ideology with the Palestinian Hamas group that has ruled the Gaza Strip since 2007, declaring: "ISIS and Hamas are branches of the same poisonous tree."

Hanan Ashrawi, an executive member of the Palestine Liberation Organization, called Netanyahu's comments a "blatant manipulation of facts" and accused him of using "hate language."

Netanyahu also drew a parallel with Nazis, saying that while they believed in a "master race", ISIS believes in a "master faith."

- 'Brazen lies' -

Netanyahu took to the podium of the world's largest diplomatic gathering, saying he wanted to expose "brazen lies" after Palestinian president Mahmud Abbas last week accused Israel of waging a 50-day "war of genocide" in Gaza.

In his address on Friday, Abbas called for an end to Israeli occupation, accused Israel of "genocidal crimes" during the Gaza war and said Palestinians living under Israeli rule faced a future of "apartheid".

Netanyahu offered a rebuttal, saying Israeli forces who fought in Gaza "deserve not condemnation but admiration" and said Israel was being "demonized."

"In what moral universe does genocide include warning the enemy civilian population to get out of harm's way or ensuring that they receive tons of humanitarian aid?" he asked.

Abbas' speech also drew a sharp rebuke from the United States, which said it contained "offensive characterizations" and undermined peace efforts.