Germany urged world powers to show a united front when they meet on Tuesday to discuss new punitive measures against Iran for its nuclear work but said a deal to ratchet up sanctions was not guaranteed.
Foreign ministers from the five permanent UN Security Council members and Germany are meeting in Berlin to try to overcome differences on a third sanctions resolution meant to punish Iran for defying demands that it stop enriching uranium.
Iran reiterated on Tuesday that new international sanctions would not stop it from pursuing its "legitimate and legal rights" to a nuclear program.
Russian and Chinese opposition to tougher sanctions has hardened since a US intelligence report last month said Iran halted its nuclear weapons program in 2003 and the powers remain divided over how far a new resolution should go.
"I can't promise we will come out of this meeting with an agreement on how to proceed in the UN Security Council, that we will agree on the text of a resolution and I can't tell you what new sanctions will look like," Foreign Minister Frank-Walter Steinmeier told ARD public television on Tuesday.
"But I am very optimistic that we can get results and show Iran that our concerns cannot be ignored, that the international community, including Russia and China, is united," he said."
The West suspects Iran is secretly pursuing an atomic bomb, but Iran says its atomic program is for civilian energy uses.
The ministers from Britain, China, France, Germany, Russia and the United States are due to meet in Berlin at 4.30 pm (1530 GMT) and will hold a news conference at 6 pm (1700 GMT).
They last met to discuss Iran in September in New York and have made little tangible progress since then.
'Critical moment'
Washington has spearheaded the drive for more punitive measures and is keen to ensure a new resolution tightens the noose on more Iranian state banks.
Russia and China, both commercial partners of Iran, are resisting, although diplomats believe both countries would accept a watered-down resolution.
Ahead of the meeting, China cautioned that the standoff, which dates back to 2002, had reached a "critical moment."
"China hopes the international community including Iran will make joint efforts for the resumption of talks so the issue can be resolved properly and comprehensively," a Chinese Foreign Ministry spokesman said on Tuesday in Beijing.
International Atomic Energy Agency (IAEA) director Mohamed ElBaradei won agreement from Iran this month to answer outstanding questions about its past covert nuclear work within four weeks.
European countries may prefer to wait until that deadline expires before moving ahead with a new resolution, one European official involved in the negotiations said.
A senior European diplomat said Tuesday's talks would ultimately depend on whether Russia and China were ready and willing to forge a compromise.
"They agree in principle that we need some kind of resolution. That's a start. But there will be haggling over what to include," he said, requesting anonymity.