Diplomatic efforts between the United States and Iran have stalled over disagreements on uranium enrichment limits, with negotiations in Pakistan ending without agreement.

The talks, held in Islamabad with Pakistan acting as facilitator, broke down over competing proposals on how long Iran should restrict enrichment.

Washington reportedly proposed a 20-year suspension in exchange for sanctions relief, while Iran rejected a proposed five-year moratorium.

Uranium enrichment, which increases the concentration of uranium-235 used in nuclear fuel, remains the central issue in long-running tensions between the two countries. According to the International Atomic Energy Agency (IAEA), low-enriched uranium is used for civilian energy, while higher levels can approach weapons-grade material above 90 per cent.

Iran is believed to hold around 440kg of uranium enriched to 60 per cent, a level that significantly shortens the technical path to weapons-grade enrichment, IAEA chief Rafael Grossi told Al Jazeera in March. Some of the material is believed to be stored at underground sites, including Isfahan and Natanz.

Iran says its nuclear programme is peaceful, while the United States and Israel accuse it of seeking nuclear weapons and have imposed sanctions. A 2015 nuclear deal, the Joint Comprehensive Plan of Action (JCPOA), limited Iran’s enrichment to 3.67 per cent in exchange for sanctions relief, but the US withdrew in 2018 under President Donald Trump.

Since then, Washington has demanded tighter and longer-term restrictions, while Iran has resisted any dismantling of its programme.

Analysts cited by Al Jazeera said the dispute over duration reflects political bargaining rather than technical necessity, with both sides seeking stronger terms than those in the 2015 agreement.

After the talks, US Vice President JD Vance said “the ball is in the Iranian court,” stressing that any deal must include mechanisms preventing Iran from acquiring a nuclear weapon.