A top Russian official made a last-ditch effort to save talks over Iran's nuclear program from collapse Tuesday, holding a meeting with Iran's chief envoy.

But diplomats said the negotiations remained deadlocked as they went into a second and possibly final day, with the presidents of the United States and Russia urging Iran to agree to curb nuclear activities that could be turned toward arming warheads and Iran demanding a lifting of sanctions crippling its oil industry.

Russia stepped into its role of trying to mediate between Iran and Western nations.

Sergei Ryabkov, Russia's deputy foreign minister and head of the presidential security council, met ahead of Tuesday's session with top Iranian negotiator Saeed Jalili, said one of the diplomats. That followed a Monday meeting between Jalili and Nikolai Patrushev, former head of Russia's counterintelligence agency.

The diplomat, like others who spoke to reporters, demanded anonymity because he was not authorized to comment publicly on the closed talks.

Diplomats from several nations meeting with Iran in Moscow depict the talks as significant. They say it could be the last in a series. And if negotiators fail to make headway in persuading Tehran to stop higher-grade uranium enrichment, it's unclear if or when new talks would occur.

Expectations were restrained after Monday's meeting ended on a downbeat note, with diplomats saying Iran had toughened its conditions in exchange for considering six-power demands that it stop enriching from low levels to higher purities closer to the consistency needed to arm nuclear missiles.

Tehran insists it is enriching only to make reactor fuel and medical isotopes. But it has refused foreign offers of fuel and is stonewalling a U.N. probe into suspicions that it secretly worked on atomic arms -- allegations it strenuously denies. It dismisses U.N. and other international sanctions meant to pressure it into stopping enrichment, which can also turn uranium into the core of a nuclear weapon, saying it has a right to do so under international law,

U.S. President Barack Obama and Russian President Vladimir Putin jointly urged Iran to show flexibility at the Moscow talks ahead of Tuesday's session.

"We agree that Iran must undertake serious efforts aimed at restoring international confidence in the exclusively peaceful nature of its nuclear program," they said on the sidelines of the meeting of G-20 nations in Mexico.

But Iran said the onus was on its six negotiating partners -- the United States, Russia, China, Britain, France and Germany -- to make the first move and agree to Tehran's demands. A member of the Iranian delegation, who demanded anonymity because he was not the official spokesman for his side, said if the six accept Iran's conditions "there will be a big progress in a short period of time.

"But if they pursue the path they've been following, any progress in the talks will be stalled," he told reporters.

The talks are being hosted by the EU's top foreign policy official, Catherine Ashton. Her spokesman, Michael Mann, spoke of "pretty tough" going, with Iran presenting arguments and objections that went over ground already covered in two previous inconclusive meetings in April and May.

The six nations had asked the Iranians to respond concretely to their demand to curb higher-level enrichment. While Mann said that topic was "addressed," he refused to qualify whether the Iranian response met the expectations of the world powers.

While the Islamic Republic has previously mentioned lifting sanctions or staying pending ones, one of the diplomats said Monday's request for "sanctions relief" was the most direct to date. That appeared to reflect the mounting pain caused by accumulating sanctions, particularly international embargoes on Iran's oil sales.

In addition to longer-term U.N. and other sanctions, Tehran is now being squeezed by the widening international embargo on its oil sales, which make up more than 90 percent of its foreign currency earnings.

Sanctions levied by the U.S. have already cut exports of Iranian crude from about 2.5 million barrels a day last year to between 1.2 and 1.8 million barrels now, according to estimates by U.S. officials. A European Union embargo on Iranian crude that starts July 1 will tighten the squeeze.

Comments by Ali Bagheri, the No. 2 on the Iranian delegation, reflected the gap between the two sides' priorities.

"We elaborated in detail ... the illegality of referring Iran's nuclear issue to the U.N. Security Council and issuance of U.N. Security Council resolutions," he told reporters, referring to Security Council demands -- enforced by sanctions -- that Iran stop enriching.

The six nations formally are only prepared to ease restrictions on airplane parts for Iran's outmoded, mostly U.S.-produced civilian fleet and are offering technical help with aspects of Iran's nuclear program that cannot be used for military purposes.

While not budging on lifting existing sanctions or those already decided upon, diplomats familiar with the talks told the AP that the six are also prepared to guarantee that no new U.N. penalties will be enacted if Tehran compromises enough. The diplomats demanded anonymity because that possible offer has not yet been formally made.

For Iran, the main formal demand remains international recognition of its right to enrich and related issues -- with increasing emphasis on sanctions relief.

They six also want Fordo, the underground Iranian facility where most of the higher-level enrichment is taking place, shut down and for Iran to ship out its higher-grade stockpile. Fordo is of special concern because it might be impervious to air attacks, a possible last-resort response to any Iranian bomb in the making.