Iran has not demonstrated sufficient flexibility in nuclear talks with six world powers aimed at ending a 12-year standoff with the Islamic Republic over its atomic ambitions, France and Britain said on Thursday.

The remarks, at the United Nations, came just after the completion of another inconclusive round of negotiations in Geneva this week between Iranian officials and the United States, Britain, France,Germany, Russia and China.

"In spite of insufficient flexibility demonstrated at this stage by Iranian negotiators, we'd like to believe that Iran does seek a long-term agreement," senior French diplomat Philippe Bertoux told the UN Security Council.

"We would expect that Iran takes strategic choices and courageous decisions" in upcoming rounds of negotiations, he added.

Senior British diplomat Michael Tatham echoed his remarks, urging Iran to be more flexible.

Iran rejects Western allegations that it is seeking the capability to produce atomic weapons. Iran and the six signed an interim deal in November 2013 and are seeking a long-term agreement that would end sanctions in exchange for curbs on Tehran's nuclear programme.

Iran and the United States, France, Germany, Britain, China and Russia last month failed for the second time this year to meet a self-imposed deadline to resolve the standoff, extending the talks for seven more months.

Iran's deputy foreign minister, Abbas Araqchi, said on Wednesday he had "very useful and helpful" nuclear negotiations with major powers in Geneva.

The French UN mission posted on Twitter on Thursday that "new ideas submitted during the talks in Vienna deserved careful consideration by the P5 + 1 members." Most of the nuclear negotiations have been held in Vienna.

Western officials say Iran has not compromised on major sticking points, including the size and scope of its future uranium enrichment program and the speed of ending sanctions.

Deputy US Ambassador David Pressman said Washington would not talk with the Iranians indefinitely without results.

"While we continue to believe that the best way to achieve our goals is thorough diplomacy, we are not going to sit at the negotiating table forever," he said.

He said the 15-nation Security Council's sanctions committee should continue monitoring implementation of UN sanctions.

He also referred to a recent report by the Iran Panel of Experts, which oversees sanctions compliance, that said Tehran was continuing to skirt sanctions by attempting to procure banned nuclear technology.

"Recent reporting by the Panel of Experts reminds us why this is so important," Pressman said. "We know that Iran is still trying to procure sensitive technology. We know Iran is still smuggling arms in violation of (the UN arms embargo on it)."

He said Iran's illegal shipments of weapons to Syria, Hezbollah militants in Lebanon and Iraq had a destabilising effect. - Reuters