Iran is preparing a counter-proposal after rejecting the U.S. nuclear deal, prompting President Trump to caution that "time is running out."

Iran's supreme leader Ayatollah Ali Khamenei said Wednesday a US proposal for a nuclear agreement was against the national interest, amid sharp differences over whether Tehran can continue to enrich uranium.

President Vladimir Putin told U.S. President Donald Trump that he was ready to use Russia's close partnership with Iran to help with negotiations over Iran's nuclear programme, the Kremlin said on Thursday.