Sanctions against Iran must "be toughened” unless Tehran negotiates seriously over its controversial nuclear programme, France has warned.French Foreign Minister Laurent Fabius said the EU should fully apply a pre-agreed oil embargo from 1 July.He was speaking after the latest round of talks between six world powers and Iran ended in Moscow with no clear breakthrough.Tehran denies Western claims it is seeking to develop nuclear weapons.After the two days of talks in Moscow, EU foreign policy chief Catherine Ashton admitted "significant gaps” remained after "tough and frank” exchanges in the Russian capital. Chief Iranian negotiator Saeed Jalili described the negotiations as "serious”, but reiterated that enriching uranium was Iran’s inalienable right. The talks between Iran and the so-called P5+1 - the US, China, Russia, France, the UK and Germany - followed other recent inconclusive rounds in Istanbul and Baghdad.Ms Ashton said the sides had agreed to continue talks on a technical level on 3 July in Istanbul before deciding on whether to hold further high-level talks.Mr Jalili agreed this expert-level meeting needed to be held. "The expert meeting could provide clear ideas for the talks,” he said. "Today they [the world powers] are facing a great test in order to obtain the confidence of the Iranian people,” he told reporters. The P5+1 are demanding that Iran suspend enrichment of uranium to 20%, close down an underground enrichment facility near the city of Qom and export its stockpile of 20%-enriched uranium - measures described by one Western diplomat as "stop, shut and ship”.In return, the world powers say they are prepared to start by offering help with nuclear safety measures.But Iran wants the West to lift sanctions, including an EU oil embargo and US measures against Iran’s central bank.It says its "non-negotiable” right to enrich uranium must be recognised. It has denounced UN Security Council resolutions against it as illegal.