Intense negotiations were due to resume Wednesday between Iran and world powers, seeking a landmark breakthrough over Tehran's nuclear programme that also satisfies hardliners in Washington, Israel and the Islamic republic.
The talks come amid heightened Middle East tensions, with Iran's foreign minister accusing Israel of trying to "torpedo" the process after twin suicide bombings killed at least 23 people outside its embassy in Beirut on Tuesday.
Tehran's foreign ministry has blamed Israel and its "mercenaries".
Israel has denied the claims and its Prime Minister Benjamin Netanyahu was due to take his tireless campaign against a deal with Iran to Moscow on Wednesday in talks with Russian President Vladimir Putin.
Iranian Foreign Minister Mohammad Javad Zarif was nonetheless upbeat about chances of a deal in Geneva, 10 days after talks between Iran and the United States, China, Russia, Britain, France and Germany -- the P5+1 -- failed to result in an accord.
"I think there is every possibility for success," Zarif, who also posted a conciliatory but defiant YouTube message online on Tuesday, said on a stopover in Rome.
But US President Barack Obama, fresh from seeking to dissuade lawmakers from imposing new sanctions on Iran, was more cautious: "I don't know if we will be able to close a deal this week or next week."
British Prime Minister David Cameron's office said Tuesday after he also phoned Rouhani -- the first such contact in a decade -- that both leaders agreed it was "important to seize the opportunity".
The P5+1 powers want a "first phase" deal whereby Iran freezes the most sensitive parts of its nuclear activities while a long-term accord is hammered out.
But the question is whether Iran, seeking an easing of UN, US and EU sanctions that have more than halved the country's lifeblood oil exports, will accept what it is being offered in return.
On the table in Geneva is only a "limited, temporary, target and reversible" relief package that a senior US official said "will not come anywhere near helping Iran escape the hole that we've put them in."
"We will maintain the sanctions as long as we are not certain that Iran has definitively and irreversibly renounced its military programme to obtain nuclear weapons," French President Francois Hollande said in Israel on Monday.
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If his "charm offensive" fails, Rouhani, whose election this year has raised big hopes of a breakthrough and an entente with the West, risks losing the support of supreme leader Ayatollah Ali Khamenei, experts say.
Right to enrich
Uranium enrichment is the main worry for the international community since enriched uranium has civilian uses -- Tehran's stated aim -- but also can go in a bomb.
Iran already has enough for several bombs if it chose to enrich further to weapons-grade, a "breakout" that -- for now -- would be detected by the International Atomic Energy Agency UN watchdog.
Hardliners in Israel and the United States want all enrichment to stop -- as multiple UN Security Council resolutions have demanded -- and Iran's entire stockpile of enriched uranium removed.
But the P5+1 are happy with the suspension of enrichment to medium levels and for stockpiles of material enriched to this level dealt with, as well as more IAEA inspections and a halt to work at the Arak reactor.
Israel, where many see the country's very existence at risk by a nuclear-armed Iran and which is widely assumed to have nuclear weapons itself, has refused to rule out bombing Iran.
"The P5+1 are giving Iran a great deal, Iran is giving practically nothing. That's a bad deal," Netanyahu told Germany's Bild newspaper.
Obama meanwhile on Tuesday sought again to talk US lawmakers out of piling more sanctions on Iran, something which could quickly scupper diplomatic efforts and prompt Iran to expand its nuclear activities further after the recent pause.
"There is a real possibility that Congress could throw a monkey wrench into the diplomacy," Mark Fitzpatrick from the International Institute for Strategic Studies told AFP.
On Tuesday Obama met with key senators at the White House, telling them to save their "robust" new sanctions for if Iran fails "to follow through on its commitments".