Iran and six world powers were unable to reach agreement Saturday on how to reduce fears that Tehran might use its nuclear technology to make weapons, extending years of inconclusive talks and adding to concerns that the diplomatic window on reaching a deal may soon close.

Expectations that the negotiations were making progress rose as an afternoon session continued into the evening. But comments by the two sides after they ended made clear that they fell far short of making enough headway to qualify the meeting as a success.

“What matters in the end is substance, and ... we are still a considerable distance apart,” Catherine Ashton, the European Union’s head of foreign policy, said at the end of the two-day talks.

Ashton, the convener of the meeting, said negotiators would now consult with their capitals. She made no mention of plans for new talks, another sign that the gap dividing the two sides remains substantial. She said she would talk with chief Iranian negotiator Saeed Jalili by telephone over further steps.

Jalili spoke of “some distance between the positions of the two sides.” He suggested that Iran was ready to discuss meeting a key demand of the other side — cutting back its highest-grade uranium enrichment production and stockpile — but only if the six reciprocated with rewards far greater than they are now willing to give.

While no breakthrough had been expected, the lack of forward movement in international negotiations that started a decade ago was certain to increase concerns that diplomacy was ineffective as a tool to stop Iran from moving toward nuclear-weapon making capacity.

Israel is most worried. It has said Iran is only a few months away from the threshold of having material to turn into a bomb and has vowed to use all means to prevent it from reaching that point.

The U.S. has not said what its “red line” is, but has said it will not tolerate an Iran armed with nuclear weapons.

“The Iranians are using the round of talks to pave the way toward a nuclear bomb,” said Yuval Steinitz, the Israeli minister for intelligence and strategic affairs, in a text message to reporters.