The five permanent U.N. Security Council members and Germany agreed in September to delay sanctions against Iran until the end of November, pending reports on an investigation by the U.N. nuclear watchdog and a European Union mediation effort.
The states decided that if the reports by the International Atomic Energy Agency (IAEA) and EU mediator Javier Solana did not show "a positive outcome," they would agree on more sanctions against Iran and put it to a vote in the Council.
Solana said his last meeting with Iran's top nuclear negotiator on Friday was disappointing. The IAEA report found Iran was cooperating, but not proactively, making it likely that Western states will resume their push for sanctions.
In previous meetings Russia and China, which have strong trade ties with Iran, have agreed only to the mildest measures backed by Britain, the United States and France.
Tehran denies Western charges it is pursuing nuclear weapons under the cover of a civilian atomic programme. It says it only wants to generate electricity but its failure to allay international fears has prompted two rounds of U.N. sanctions.
The Security Council has demanded Iran suspend uranium enrichment, a process that can produce fuel for power plants or, potentially, nuclear weapons. Tehran has refused and said it has a right to the sensitive technology.
"LOGICAL BEHAVIOUR"
Saturday's talks, which began shortly after 8 a.m. EST and were due to last until around 11 a.m. EST, are the first time the six world powers will discuss the IAEA report and the outcome of Solana's talks.
An EU official will give a report on Solana's mediation effort, repeating what Solana said in public on Friday, Solana's spokeswoman said.
Asked how Iran would respond if Solana gave a negative report, Iran's top nuclear negotiator Saeed Jalili said on his return from London:
"We expect a positive report but, anyhow, we will react accordingly to safeguard our nation's rights as a signatory of the NPT (nuclear Non-Proliferation Treaty) within the framework of the NPT."
He did not say what action Iran would take if a third U.N. sanctions resolution was passed. Iranian officials have said previously Iran could review its cooperation with the U.N. nuclear watchdog, the International Atomic Energy Agency (IAEA).
Asked if he expected another U.N. resolution against Iran, Jalili said: "Currently there is a positive atmosphere based on the (IAEA's) report and our logical behavior within the framework of the NPT but there are a few powers that had pre-judgments to act against our logical behavior."
Iran is working with the IAEA on a plan agreed in August to answer outstanding questions about its nuclear programme.
(Additional reporting by Reza Derakhshi in Tehran and Ingrid Melander in Brussels)
