Israel's Prime Minister Benjamin Netanyahu denounced President Barack Obama's drive for a nuclear accord with Iran on Tuesday, warning Washington is paving Tehran's path to a bomb.

After Netanyahu delivered an impassioned address to the US Congress, an exasperated Obama retorted that the Israeli leader had no plan of his own to contain the Iranian threat.

And, with US Secretary of State John Kerry in talks with his Iranian counterpart in Switzerland, the "P5+1" world powers pressed on for an accord that would limit Iran's nuclear options while loosening economic sanctions.

Netanyahu's party faces a close-fought Israeli parliamentary election in two weeks' time and he has been accused of imperilling Israel's close ties with Washington for his own political gain.

But, in a barnstorming speech to a packed joint session of Congress, he pulled no punches in his denunciation of the accord that he expects Tehran to be offered before the month is out.

"That deal will not prevent Iran from developing nuclear weapons," he declared. "It would all but guarantee that Iran gets those weapons, lots of them."

Obama did not watch the speech, having arranged a video conference with European leaders on the crisis in Ukraine, but afterwards he gave the Israeli leader's argument short shrift.

"I am not focused in the politics of this. I am not focused on the theater," Obama said. "As far as I can tell, there was nothing new.

"On the core issue, which is how to prevent Iran from obtaining a nuclear weapon which would make it far more dangerous, the prime minister did not offer any viable alternatives."

"We don't yet have a deal (with Iran)," Obama added. "But if we are successful then, in fact, this will be the best deal possible to prevent Iran from obtaining nuclear weapons."

- 'Spark nuclear arms race' -

While dozens of Obama's Democrats stayed away from the Congress address, many more lawmakers from both sides of the aisle attended and gave Netanyahu several warm standing ovations.

"I deeply regret that some perceive my being here as political. That was never my intention," he insisted.

"We appreciate all that President Obama has done for Israel."

But, despite his conciliatory opening words, Netanyahu's speech built quickly into a thorough denunciation of Obama's efforts to strike an accord with Iran.

"This deal has two major concessions. One, leaving Iran with a vast nuclear program. Two, lifting the restrictions on that program in about a decade," he said.

"That's why this deal is so bad. It doesn't block Iran's path to the bomb. It paves Iran's path to the bomb."

He cited the strength of Iranian-backed factions in Lebanon, Yemen, Syria and Iraq as evidence of Tehran's regional ambitions, and recalled threats against his own people.

Thus, he said, no nuclear deal should weaken economic sanctions against Iran until its government ends its aggressive regional stance and frequent threats against Israel.

"A deal that is supposed to prevent nuclear proliferation would instead spark a nuclear arms race in the most dangerous part of the planet," he warned.

- Mixed reviews at home -

Netanyahu received several standing ovations in the chamber and good reviews from many US foreign policy hawks, but afterwards some Democrats accused him of "fear-mongering."

Leading House Democrat Nancy Pelosi said she had been "saddened by the insult to the intelligence of the United States."

In Israel, the voters who will be called to give a verdict on Netanyahu's governing coalition in two weeks were divided.

Some watching the speech in Jerusalem told AFP they had been impressed by the respect he had been accorded in the chamber, while others were concerned that he had recklessly interfered in the domestic politics of a vital ally.

Iran denounced Netanyahu's intervention -- even as Tehran's envoys sat with Kerry in Montreux to hammer out the nuclear agreement ahead of a March 31 deadline.

Foreign ministry spokeswoman Marzieh Afkham accused Netanyahu of spreading lies about Iran's nuclear program, which Tehran insists is peaceful.

And she dismissed the speech as repetitious, boring and a "sign of weakness."

European Union foreign policy chief Federica Mogherini also implicitly criticized Netanyahu, warning against "spreading fears" and promising that a deal is getting close.

What an Iran nuclear deal could look like
Montreux, Switzerland (AFP) March 3, 2015 - Israeli Prime Minister Benjamin Netanyahu Tuesday sought to shoot down a nuclear deal being negotiated behind closed doors between Iran and world powers, warning it would leave Tehran free to develop atomic weapons.

But he did not reveal -- as his aides had warned -- specific details of the agreement shared with Israel in classified briefings by US officials.

Few insights into the comprehensive accord under discussion since November 2013 have leaked out, with officials remaining tightlipped to protect the high-stakes negotiations.

But US officials did lay out late last week some of what they called the "bottom lines" of any deal, without going into specifics.

Here are some of the possible contours of a deal:

The goal:

To reach a verifiable, comprehensive agreement that limits Iran's ability to harness enough fissile material to build a nuclear bomb. In return, the international community would initially ease and then lift all sanctions imposed on the Islamic Republic.

Breakout time:

World powers want to cut Iran's ability to build an atomic weapon to a "one-year breakout time." That would mean Tehran would need at least 12 months to be able to produce enough fuel for a nuclear bomb.

The assumption is that the international community would have enough time to detect such a move -- and could seek to strike or destroy the facilities.

This year-long breakout time would stay in place for the length of the deal, which US officials have said they want to be in "double-digits".

Many assume that this figure is pointing to a deal lasting about 10 years, but US officials have refused to comment.

Enrichment:

This is one of the trickiest issues. Iran currently has about 19,000 centrifuges. About 10,200 centrifuges are in operation, used for spinning uranium gas at supersonic speeds to make it suitable for power generation and medical uses but also, at high purities, for a bomb.

According to documents leaked by the Israelis, and deemed accurate by non-proliferation experts, the US wants Tehran to reduce its total number of centrifuges to between 6,500-7,000.

Under the 2013 interim deal, Iran has halted production of 20 percent highly enriched uranium and eliminated or diluted much of its stock down to just five percent in return for limited sanctions relief.

Negotiators now seek to enshrine that agreement and cut Iran's stockpile of low-enriched uranium gas. A senior US official said Tehran would have to "significantly reduce" both the number of centrifuges and its uranium stocks.

There may also be a proposal allowing Iran to ship its uranium gas to Russia, which would convert it to fuel rods for the Bushehr nuclear plant which Moscow helped build.

Nuclear plants:

A senior US administration official said last week Iran should not be allowed to develop weapons-grade plutonium at its unfinished Arak reactor.

Plutonium can be used as an alternative fissile material to highly-enriched uranium.

"We're discussing how Iran can convert that Arak reactor to serve a different purpose," the official said.

Iran should also not use its Fordo nuclear plant to enrich uranium, which would leave only its Natanz plant capable of enriching uranium.

Monitoring:

A tough inspection programme using UN watchdog, the IAEA, is a cornerstone of any deal to ally any fears that Iran could covertly develop a nuclear arsenal. The US bottom line is that Iran must agree to unprecedented inspections of both nuclear and production facilities as well as uranium mines and mills and suspect sites.

Sanctions:

Iran wants all sanctions imposed by the US, European Union and United Nations to be lifted. But world powers have refused, talking instead about a phased, gradual easing of the measures. Experts say untangling the sanctions -- from those also imposed for Tehran's alleged terror activities for example -- could in fact be proving one of the most difficult tasks.

Civilian nuclear programme:

Iran has always denied seeking a bomb, saying its nuclear programme is for peaceful civilian energy purposes.

Under the Joint Plan of Action agreed in November 2013, the global powers known as the P5+1 acknowledged that a comprehensive deal "would enable Iran to fully enjoy its right to nuclear energy for peaceful purposes" in line with the nuclear Non-Proliferation Treaty (NPT).