During a phone call with President Donald Trump this week, Russian President Vladimir Putin suggested that Iran's enriched uranium be moved to Russia as part of a deal to end the war.
Trump has rejected the offer, Axios sources have reported.
Seizing Iran's 450 kilograms of 60% enriched uranium — which can be converted into weapon-grade uranium within weeks and is enough for more than 10 nuclear bombs — is one of the key military goals of the US and Israel.
"This is not the first time such an offer has been made. It has not been accepted. "The U.S. position is that we need to see that the uranium is secured," a U.S. official told Axios.
Russia made similar proposals during the U.S.-Iran nuclear talks last May - before the U.S. and Israel attacked Iran's nuclear facilities in June - and in the weeks leading up to the current war.
In theory, Putin's proposal could facilitate the dismantling of Iran's nuclear stockpile without the intervention of U.S. or Israeli troops on the ground.
Russia is already a nuclear power and has in the past stored low-enriched uranium from Iran under the 2015 nuclear deal, making it one of the few countries with the technical capacity to accept the material.
Putin laid out several ideas for ending the U.S.-Iran war during his call with Trump on Monday. The uranium proposal was one of them.
In the last round of negotiations before the war, Iran rejected the idea of a transfer and proposed that the uranium remain in its own facilities under the supervision of the International Atomic Energy Agency.