Lammy met with Rubio and US special envoy Steve Witkoff in Washington one day before he was due in Geneva for talks with Iranian Foreign Minister Abbas Araghchi alongside his French, German and EU counterparts.

The meetings come as European countries call for de-escalation in the face of Israel's bombing campaign against Iran's nuclear program -- and as US President Donald Trump weighs up whether or not to join the strikes against Tehran.

"The situation in the Middle East remains perilous. We are determined that Iran must never have a nuclear weapon," Lammy said in a statement released by the UK embassy in Washington.

"We discussed how Iran must make a deal to avoid a deepening conflict. A window now exists within the next two weeks to achieve a diplomatic solution," Lammy said.

"Tomorrow, I will be heading to Geneva to meet with the Iranian foreign minister alongside my French, German and EU counterparts. Now is the time to put a stop to the grave scenes in the Middle East and prevent a regional escalation that would benefit no one."