US President Donald Trump launched the war on February 28 under the pretext that Iran was developing an atomic bomb -- which it denies -- and vowed to never let the Islamic republic get its hands on a nuclear weapon.

Vice President JD Vance left negotiations with Iran over the weekend empty-handed, with sticking points including freeing up the Strait of Hormuz and Iran's nuclear program.

Media outlets reported that Washington asked Tehran to agree not to enrich uranium for 20 years, citing officials close to the negotiations held in Islamabad on Saturday.

The 20-year pause would be accompanied by sanctions relief, the Wall Street Journal reported.

Iran in turn proposed to suspend its nuclear activity for five years, according to The New York Times.

The reported proposals would be a watered-down version of Trump's previous demands that Iran permanently give up its nuclear ambitions.

In 2018, Trump pulled out of what he called a "one-sided" pact that lifted sanctions on Iran in exchange for guarantees that Tehran would not make an atomic bomb.

Vance said that Washington had made its red lines clear in talks with Tehran, and that the "ball is in the Iranian court" now.

"There are two things in particular where the president of the United States really said we have no flexibility," Vance said on Monday.

"It's one thing for the Iranians to say that they're not going to have a nuclear weapon. It's another thing for us to put in place the mechanism to ensure that's not going to happen," he added.

Iran has previously ruled out restrictions on its right to enrich uranium in what it insists is a civilian nuclear program.

Earlier on Monday, Israeli Prime Minister Benjamin Netanyahu said Vance had told him the removal of all of Iran's 60-percent enriched uranium from the country is a "central issue" for Trump.

The Washington delegation also wanted to ensure "that there is no further enrichment in the years ahead -- even decades ahead -- no enrichment within Iran," Netanyahu added.

Russia has offered to hold Iran's enriched uranium safely as part of any deal on Monday.

"The offer still stands, but has not been acted upon," Kremlin spokesman Dmitry Peskov told reporters.