The West fears Russia has traded nuclear secrets with Iran in exchange for ballistic missiles to attack Ukraine, according to reports.
This alarming development was discussed by Joe Biden and Sir Keir Starmer at the White House on Friday.
Intelligence sources suggest Iran's nuclear programme may be nearing completion with Russian assistance, The Sun reported.
US and UK intelligence revealed Putin has received Fath-360 ballistic missiles from Iran's Supreme Leader.
US Secretary of State Antony Blinken warned: "Russia is sharing technology that Iran seeks — this is a two way street — including on nuclear issues, as well as some space information."
In response, Iran Air has been banned from UK airspace and Russian shipping firms face sanctions.
The summit also addressed allowing Ukraine to use UK-made Storm Shadow missiles against Russian targets, but no decision was announced.
Boris Johnson, visiting Ukraine, urged swift action: "There is no conceivable case for delay. The only person who fears escalation is Vladimir Putin and every day that goes by is a lost opportunity to save lives and bring about a just conclusion to this war."
Sir Keir is set to discuss the Storm Shadow issue in Italy, with plans to seek French and German support.
However, Biden remains cautious about potential conflict escalation, while Putin has warned of war with NATO if long-range missiles are approved.
It comes after five former defence ministers and Boris Johnson have urged Sir Keir Starmer to allow Ukraine to use Storm Shadow missiles against Russia, even without US backing.
Grant Shapps, Ben Wallace, Gavin Williamson, Penny Mordaunt, Liam Fox and Johnson united in warning Starmer that further delay would only strengthen Putin's resolve.
