Iran's talks with global powers on curbing its nuclear-weapons program could be extended for another six months if no deal is reached by a July 20 deadline, a senior Iranian official said on Monday.

U.S. and Iranian officials held talks in Geneva on Monday to tackle ways of breaking a deadlock that has raised the likelihood that the deadline will lapse without a deal meant to head off the risk of a Middle East war over the nuclear issue.

The four-month-old round of negotiations ran into difficulty last month with each side accusing the other of making unrealistic demands, sowing doubt about prospects for a breakthrough next month.

Western officials say Iran wants to maintain a uranium-enrichment capability far beyond what is suitable for civilian nuclear power stations. Iran says it wants to avoid reliance on foreign suppliers of fuel for planned nuclear reactors and rejects Western allegations it seeks the capability to make nuclear weapons under the guise of a peaceful energy program.

Deputy Foreign Minister Abbas Araqchi spoke of a possible extension to the talks in remarks in Geneva to Iranian media after meetings with senior U.S. officials and the European Union's deputy chief negotiator.

"We hope to reach a final agreement (by July 20) but, if this doesn't happen, then we have no choice but to extend the Geneva deal for six more months while we continue negotiations," Araqchi told Iran's state news agency IRNA."

The United States said on Saturday it would send its No. 2 diplomat, Deputy Secretary of State Bill Burns, and Under Secretary of State Wendy Sherman, the primary U.S. negotiator with Iran, to Geneva to meet a delegation led by Araqchi.

Burns led secret U.S.-Iranian negotiations that helped yield an interim nuclear agreement between Iran and the major powers on Nov. 24, allaying fears of a wider Middle East war.

U.S. State Department deputy spokeswoman Marie Harf said the U.S. and Iranian delegations met for over five hours in Geneva for what she described as "wide-ranging discussions".

"They will reconvene tomorrow morning and expect to meet all day," she told reporters in Washington, adding that the consultations were taking place ahead of the next round of Vienna negotiations scheduled for June 16-20.

"We are at a critical juncture in the talks," Harf said. "We don't have very much time left. We think we've made progress during some rounds but as we said coming out of the last one we hadn't seen enough made, we hadn't seen enough realism."

A French diplomatic source said officials from France and Iran would meet on Wednesday to discuss the Vienna negotiations. And Russian officials will have talks with the Iranians in Rome on Wednesday and Thursday, according to Iranian media.

A second senior Iranian official, Takht Ravanchi, was quoted as saying that putting an end to sanctions was one of the issues discussed during the bilateral session with the Americans.

Iran, the United States, Britain, France, Germany, Russia and China included the July 20 deadline to reach a comprehensive agreement in the text of the Nov. 24 preliminary deal.

That pact, under which Iran shelved some sensitive nuclear work in exchange for limited relief from western sanctions, gave scope for a six-month extension if needed to reach a final settlement that would end sanctions and remove the threat of war.