It is the first meeting of permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany, since Iran failed on August 31 to honour a Council deadline to halt uranium enrichment, the process which makes nuclear reactor fuel but also atom bomb material.
The six are offering Iran talks on a package of trade, security and technology benefits if it freezes enrichment.
Chicago-based non-proliferation expert Gary Samore said the six six nations' foreign ministry political directors were “going to talk about sanctions, and my guess is that they're going to come up with not very much.”
A Western diplomat agreed, saying the gathering was "not a decision-making meeting. It is the first conversation on where we go from here."
Washington leads international concerns that Iran is covertly trying to develop nuclear weapons, something Tehran denies, as well as the push for sanctions.
But Iranian allies and major trading partners Russia and China are reluctant to take such measures, fearing they will worsen the current confrontation or even lead to war.
EU meeting postponed
The Berlin meeting comes after an EU-Iran effort to kickstart negotiations over Tehran's nuclear program and give it a chance to avoid UN sanctions were postponed at the last minute in Vienna yesterday.
EU foreign policy chief Javier Solana and Iranian negotiator Ali Larijani had tentatively planned to meet in Vienna ahead of the Berlin talks, but it was broken off with Iranian ambassador Ali Asghar Soltanieh telling AFP there was "no particular reason" for the delay.
Spain talks
Mr Larijani is due to visit Spain today for talks with Foreign Minister Miguel Angel Moratinos, an official in the Spanish presidency said.
He is also expected to briefly meet Prime Minister Jose Luis Rodriguez Zapatero.
"Spain hopes to facilitate the dialogue" between Iran and European Union foreign policy chief Javier Solana over the international community's standoff with Iran concerning its disputed nuclear programme, the official said.
Spanish news agency Europa Press reported that Mr Moratinos is expected to press Mr Larijani to respond positively to an offer of economic incentives from world powers in exchange for Tehran's suspension of uranium enrichment.
Mr Larijani’s visit to Madrid coincides with one from UN Secretary General Kofi Annan, who is also due to meet Mr Zapatero and Mr Moratinos for talks today on Iran and the UN peacekeeping mission in Lebanon.
The EU said at a foreign ministers meeting in Finland on Saturday that it would give Iran extra time to show it was serious about negotiating but warned the Western bloc expected a clear response.
Sanctions
In a sign of mounting international impatience, Russian Foreign Minister Sergei Lavrov said Russia was considering support for Security Council economic sanctions against Iran.
"We will consider this from all points of view, in totality, based on our goal of not allowing the spread of WMD (weapons of mass destruction) and technology that is linked with this," state-run RIA Novosti quoted him as saying.
Mr Lavrov said however that Russia still had reservations about imposing sanctions on Iran and he underlined Moscow's opposition to military action.
But Robert Joseph, US under secretary of state for arms control and international security, said it was "essential" for the Security Council to impose sanctions on Iran, saying the prospect of Tehran with a nuclear arsenal was "intolerable."
Mr Joseph said he believed a vote on sanctions could come this month, and he expected Russia and China would sign on.
He also accused Tehran of running out the clock after the postponement of EU-Iran talks in Vienna yesterday.
State Department spokesman Sean McCormack yesterday said: "We have gotten to a point, because of Iran's behavior, that the international community frankly doesn't believe Iran when it says that it is pursuing only a peaceful nuclear energy program."
Iran hits back
Iranian President Mahmoud Ahmadinejad was defiant in talks in Tehran with UN Secretary General Kofi Annan at the weekend, saying he was prepared to negotiate on Iran's nuclear program but would not accept a suspension of enrichment first.
The campaign picked up pace after President George W. Bush on Tuesday branded Iran's leaders "tyrants" and said they must not be allowed to get nuclear weapons, "the tools of mass murder."
Iranian President Mahmoud Ahmadinejad hit back saying Bush was "nothing" compared to God's will.
"I am telling him (Bush) that all the world is threatening you since the general path that the world is taking is towards worshipping God and divinity," president Ahmadinejad told a conference in Tehran.
He urged the West to turn to God's path and said that failure to do so would tempt fate.
President Ahmadinejad also rebuffed accusations that Iranian policy constitutes a threat to the United States and again challenged US President George W Bush to a live television debate, which Washington has said is a diversion.
Later, the president told his cabinet that the debate could be held in the UN General Assembly and that Mr Bush could bring his advisers if he wished.
"We oppose the fact that America and Britain intend to impose themselves on every other nation," president Ahmadinejad said.
Khatami security
Meanwhile the US State Department is offering security for former Iranian president Mohammad Khatami during his two-week tour of the US which starts today, department spokesman Sean McCormack said.
"There's precedent for this," he said. "Former high-ranking officials travelling throughout the United States, we've done this type of thing before."
He did not, however, give the exact reasons the administration decided to offer protection to the former leader of a country the United States considers a primary backer of international terrorism.
The former Iranian leader, who has no scheduled meetings with US government officials during his two-week visit, is the most senior Iranian representative to travel to the United States since Washington broke off diplomatic relations following the 1979 storming of the US embassy in Tehran.