WASHINGTON — Iran and its negotiating partners are approaching the decisive phase in their prolonged discussions over a nuclear deal and have discussed several ways in which Iran’s enrichment capacity could be curbed to meet Washington’s demand for a one-year breakout, sources briefed on the negotiations say.

Amid a sense of urgency to see if Iran and the five permanent members of the UN Security Council plus Germany (P5+1) can reach a political agreement for a nuclear deal by the end of March or risk a likely congressional vote on new Iran sanctions, US and Iranian negotiators are approaching their bottom lines as they continue to haggle over three key elements of a final deal: enrichment capacity, sanctions relief and duration.

In recent meetings, the United States has presented Iran with a few different ways an acceptable enrichment capacity could be achieved in a final deal that would meet Washington’s requirement for a minimum one-year “breakout” — the amount of time it would take Iran to produce enough enriched uranium for a nuclear bomb. As they wait to see if Iran agrees to move forward with one of the proposed formulas, Iran is pressing for earlier sanctions relief in a deal. The two sides are also haggling over the duration of a deal, with the P5+1 reportedly asking for 15 years, and Iran for a shorter deal.

“The issues now are sufficiently narrowed and sufficiently clarified, where we're at a point where they need to make a decision,” President Barack Obama said at a press conference with German Chancellor Angela Merkel Feb. 9.