US and British diplomats have argued the west's strategy on Iran – involving a new round of measures at the United Nations – still has time to succeed, despite Tehran's failure last week to meet a UN deadline on its nuclear programme.

"The genie isn't out of the bottle yet but . . . they are working very aggressively to get it out," a US official on Monday night told the Financial Times, referring to Iran's efforts to move ahead with uranium enrichment, which can produce both nuclear fuel and weapons-grade material.