A second day of nuclear talks between Iran and world powers was continuing in Geneva as a senior Iranian negotiator played down the prospect of an immediate agreement.
Catherine Ashton, the powers' chief negotiator, held extended meetings with Iranian Foreign Minister Mohammad Javad Zarif, described by both sides as "substantial and detailed."
However, Zarif's deputy, Abbas Araqchi, told Iranian media it was unlikely the negotiations "will reach a conclusion tonight."
The talks are scheduled to end on November 22.
Zarif earlier told Iranian television that disagreements "on important issues" remained.
The Geneva talks come after an inconclusive first round held in the same city earlier this month.
The talks focus on efforts to agree upon a six-month suspension of Iran's nuclear activities in exchange for limited relief for Tehran from Western sanctions.
On November 21, U.S. Senate Majority Leader Harry Reid (Nevada) said he was committed to moving ahead with a tougher Iran sanctions bill in early December, adding to pressure on Tehran.
Reid said he supports the negotiations to curb Iran's nuclear program but without new sanctions the Iranians could prevent a successful deal.
Reid said lawmakers should expand oil sanctions and target Iranian industries supporting the nuclear program.
Araqchi had said the session was "very useful" in restoring "some confidence," but he added that "serious issues" remained. Before the session, Araqchi warned that "lack of trust" remained an obstacle because of inconclusive results at the previous round of talks earlier this month.
Ashton is leading negotiations for the six world powers: the United States, Russia, China, Britain, France, and Germany. The Iranian foreign minister heads Tehran's negotiating team.
Several world leaders sought to influence the progress of the talks on November 20 from afar.
Russian President Vladimir Putin said in Moscow he believes a solution is within reach.
"As the sextet consultations with Iran have demonstrated, the possibility is there," he said. "It exists, and Russia has an optimistic view of the current talks."
Putin spoke after meeting with Israeli Prime Minister Benjamin Netanyahu, who is visiting Moscow.
Netanyahu, who has repeatedly warned that Iran cannot be trusted, said Israel wants to see a negotiated settlement but that it must be "genuine and real."
He said the best solution is to disarm Iran the way Syria has been forced to give up its chemical weapons.
"I think this is analogous to the situation in Syria," he said. "It looks a lot like the Iranian problems that are now being decided by the countries in the Geneva conference."
Meanwhile, Iran's supreme leader, Ayatollah Ali Khamenei, said on November 20 that the Islamic republic's nuclear rights are nonnegotiable.
"We do insist that we will not step back one iota from the rights of the Iranian nation," Khamenei said.
Iran claims the right to all aspects of a nuclear-energy program, including uranium enrichment, and says its program is entirely peaceful. Western powers accuse Iran of seeking nuclear weapons capabilities under the cover of its nuclear program.
Western powers want Iran to stop enriching uranium to 20 percent, a short technical hop from weapons-grade levels. They also are concerned over a research reactor under construction at Arak in western Iran which will produce weapons-grade plutonium.
The current effort to reach a six-month suspension of Iran's nuclear activities in exchange for limited sanctions relief is intended as a confidence-building measure.
Negotiators hope a six-month agreement might then provide momentum for a later comprehensive settlement of the Iran nuclear crisis.
Based on reporting by Reuters, AFP, and AP