Six world powers and Iran missed a Tuesday deadline to reach an outline accord reining in Tehran‘s nuclear programme. The talks now go into an extra day as negotiators failed to agree upon crucial details.

The negotiators of Iran and Russia expressed optimism that an initial agreement was within reach. The preliminary deal is a major milestone towards a final accord, with an end-June deadline, that could end the 12-year-old standoff and reduce the risk of another Middle East war.

With Iran asserting its “nuclear rights” and the United States threatening to abandon negotiations, the talks have been bogged down on the issues of nuclear research, the lifting of UN sanctions and their restoration if Iran breaches the agreement.

Russian Foreign Minister Sergei Lavrov said negotiators had reached a general accord on “all key aspects”, according to Russia‘s TASS news agency. But a diplomat close to the talks denied that such an agreement had been reached. A French official said their Foreign Minister Laurent Fabius was leaving the talks and would return from France when it was “useful”.

The six powers - the United States, Britain, France, Germany, Russia and China - aim to stop Iran from gaining the capacity to develop a nuclear bomb in exchange for easing international sanctions that are crippling its economy. Iran says its nuclear programme is peaceful.

The US Congress has warned it will consider imposing new sanctions on Iran if there is no agreement this week, giving an added sense of urgency to the talks. But President Obama has threatened to veto any sanctions moved by the Republican-controlled Congress.

Russia Says Key Details Agreed up on

Lavrov was quoted by TASS as saying that the possible agreement included the International Atomic Energy Agency‘s (IAEA) control over Tehran‘s nuclear programme as well as steps to lift sanctions. He added experts would work out technical details by end-June.

One can say with enough confidence that (foreign) ministers have reached a general agreement on all key aspects of a final settlement to this issue. It will be put down in writing over the next few hours, maybe during the day.

- Sergei Lavrov, Russian Foreign Minister

White House spokesman Josh Earnest said in Washington that US negotiators would not wait until June 30 to walk away from the talks if they could not reach a preliminary political agreement.

Brent crude dropped towards $55 a barrel on Tuesday as negotiations proceeded.

Officials have said Britain, France and the United States want any removal of UN sanctions to be automatically reversible, but the Russians dislike this because it would weaken their veto power over the deal.