Iran and world powers on Tuesday failed to make headway in a second day of bruising talks in Moscow aimed at breaking an increasingly risky deadlock in the crisis over Tehran's nuclear drive.

With the United States and Israel refusing to rule out military action and Tehran facing severe economic sanctions, the price of failure in the Russian capital could be high but there was no sign of progress after five hours of talks.

Negotiators from the six world powers asked Iran to scale back its enrichment of uranium, a process which can be used to make nuclear fuel but also the explosive core of a nuclear bomb.

But Iran's chief nuclear negotiator Saeed Jalili appeared more interested in winning a pledge from the world powers guaranteeing its right to uranium enrichment and the two sides were increasingly talking at cross purposes.

"We believe we are at a crossroads. And today the opposite side has to choose a path," said a member of the Iranian delegation, asking not to be named.

"One direction is an exit from the deadlock that has existed for the last 10 years. We have been fully transparent about the detailed steps needed to take this path."

The diplomat complained that the world powers' responses in talks the day earlier had been "superficial" but did not give any comment on the progress of negotiations so far Tuesday.

The two sides opened their first plenary session after the Iranian team had bilateral discussions in the morning with the Russian delegation and the world powers held their own internal meeting to coordinate strategy.

After a pause for lunch and prayers, a new plenary session started in the afternoon. The Russian host delegation and Jalili's team then held another bilateral meeting -- whose purpose was not explained -- in his hotel room.

With time running out to make progress in the two-day Moscow round, Iranian media speculated that a new session of talks could be held at a different venue in the next weeks, possibly Beijing or Astana.

"It is possible that the talks be resumed between the two sides in another place, another time," an Iranian source told the state IRNA news agency.

The world powers are the so-called "P5+1" -- permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany -- with their negotiating team led by EU foreign policy chief Catherine Ashton.

EU delegation spokesman Michael Mann said that world powers had on Monday repeated the demand at the last session of talks in Baghdad last month that Iran stops enriching uranium to 20 percent, a degree approaching the level required in nuclear weapons.

The Islamic Republic however "brought up lots of questions and well-known positions, including past grievances," he added.

Iran has repeatedly over the last years insisted it has every right to enrich uranium but it remains to be seen if it could accept a compromise solution and enrich uranium to a lesser degree of around five percent.

The talks mark a rare show of unity between the West and Russia. After their meeting in Mexico Monday, US President Barack Obama and Russian President Vladimir Putin jointly warned Iran it must comply with its nuclear obligations.

The West has long accused Iran of seeking an atomic bomb under the guise of a civilian nuclear energy programme, a charge vehemently denied by Tehran. Host Russia has long taken a more cautious line, saying Iran must restore confidence but not explicitly accusing it of military intentions.

If Iran accepted the demands of world powers to scale down enrichment, it would in return receive some forms of peaceful nuclear energy cooperation from the West and much needed spare-parts for its embargo-hit aviation sector.