France called yesterday for tougher sanctions against Iran over its controversial nuclear programme.

"The time has come to seek firmer sanctions against Iran," secretary of state for European affairs Pierre Lellouche said on the French Jewish radio station Radio J.

Iran's supreme leader Ayatollah Ali Khamenei lashed out yesterday at the United States and Britain, labelling them Tehran's main "enemies" and warning they will fail to isolate Iran over the nuclear issue, state TV said.

Tension over Tehran's controversial nuclear programme has peaked in recent weeks after it rejected a high-profile nuclear deal brokered by the UN atomic watchdog.

World powers object to Tehran's uranium enrichment programme, as the process can be used to produce fuel for a nuclear reactor or to make an atomic bomb. Iran insists it is enriching uranium for peaceful ends.

President Mahmoud Ahmadinejad has vowed to build more uranium enrichment facilities, raising the possibility that the US Security Council could seek to pass further sanctions.

UN Secretary-General Ban Ki-Moon yesterday expressed strong concern over Iran, but said he had no details of any proposals for further sanctions.

He said Iran must "fully respect" resolutions already passed on its nuclear programme by the United Nations Security Council, according to a French translation of Mr Ban's comments on television channel TV5.

Mr Lellouche recalled yesterday that Iran has refused a UN-mediated offer to send its uranium stockpile to Russia and France to be refined for use in a medical reactor.

Tensions have risen between Tehran and Paris since Iran's election in June, notably over the detention of French academic Clotilde Reiss, who was arrested in Iran during a government crackdown after the vote.

Iran also snubbed France in October during international talks on the nuclear issue.