VIENNA - World powers and Iran on Sunday said big differences remained in talks to build a lasting agreement on the Islamic republic’s nuclear activities by the end of next week, as foreign ministers flew into Vienna to push negotiations along.“We have some very significant gaps,” US Secretary of State John Kerry told reporters on arrival. “On practically all the important issues differences persist and we have not been able to narrow them,” one of Iran’s top negotiators, Abbas Araqchi, told Al-Alam television.The unbridged positions threaten to prevent the much sought-after, historic deal being struck by its July 20 deadline, when a six-month interim accord with Iran runs out. If no agreement is reached by Sunday next week, both sides can decide to extend the interim pact for some weeks or months to keep talking. In a sign of the high stakes at play, Kerry and the foreign ministers of France, Britain and Germany flew into Vienna on Sunday to face off with Iranian counterpart Mohammad Javad Zarif. Russia and China, the other two members of the P5+1 group comprising the five permanent UN Security Council members states plus Germany, were sending only lower-ranking officials, however.The Western ministers were to use the opportunity to hold talks among themselves the deadly conflict escalating between Israel and the Palestinian group Hamas in Gaza. Calls for a ceasefire are mounting, but have so far been ignored by the warring sides.Kerry, coming directly from Afghanistan where he brokered a breakthrough to end an election crisis there, will also seek to ease a major row over spying with Germany, which saw the CIA chief in Berlin expelled from the country. Speaking about the Iran talks, Kerry said: “We need to see if we can make some progress,” adding that “it is vital to make certain that Iran is not going to develop a nuclear weapon, that their programme is peaceful”.British Foreign Secretary William Hague echoed that, saying that the differences meant “it is unlikely that there will be a quick breakthrough today”.China urged world powers and Iran “to show flexibility” in talks over the Islamic republic’s nuclear programme, after officials from both sides said big gaps remained in negotiating positions.“We urge all parties to show flexibility and political will to reach a comprehensive agreement,” Chinese deputy foreign minister Li Baodong told reporters in Vienna, where the talks are being held.French Foreign Minister Laurent Fabius said that if a deal was not struck by next Sunday, “we either extend, a so-called rollover, or we will have to say that unfortunately there is no perspective for a deal”. He added: “We don’t know yet. It’s not yet July 20. We are trying to go in the right direction.”Araqchi, in his interview with Al-Alam, said “differences have been narrowed” on “certain” other issues and “some solutions have been put forward” in the final-round negotiations, which started on July 3. But on the major divergences, “it is still not clear if we will get there,” he said.Iranian Foreign Minister Mohammad Javad Zarif said on his official Twitter account he was demanding good faith in the talks. “Trust is a two-way street,” he said, adding: “What I will engage in is a sincere effort to come to an agreement. I expect the same” from the world powers.The core sticking point of Iran’s uranium enrichment - an activity that can produce fuel for the country’s sole nuclear plant or, if further enriched, the matter for an atomic bomb - has run up against declarations by Ayatollah Ali Khamenei, Iran’s supreme leader who sets the parameters for his country’s negotiators.Khamenei on Tuesday declared that Iran wants to immensely increase its enrichment capacities for theoretical future nuclear power plants - directly challenging the P5+1’s demand they be greatly reduced.Araqchi said: “Concerning enrichment, our position is clear and rational. As the supreme guide said, the enrichment programme has been planned with the real needs of the country in mind, meaning our need to ensure reactor fuel.”On Saturday, Araqchi said Iran was ready to walk away from the talks if the world powers pushed on with “excessive” demands.The UN Security Council and the International Atomic Energy Agency (IAEA) have long been concerned about Iran’s outsized atomic activities and secret construction of two uranium enrichment sites, one of which is heavily fortified under a mountain. The Security Council has passed six resolutions demanding Iran cease all enrichment, which Tehran has defied.Despite Iranian insistence that the enrichment is exclusively for energy production and medical purposes, the P5+1 fears Tehran’s programme was aimed at attaining nuclear weapon capability.The talks are designed to limit Iran’s atomic activities so that they can only be used for civilian ends, and that any “breakout” point is pushed back years. Washington wants Iran’s uranium enrichment limited for at least 10 years, a senior US official said Saturday.In return, Iran would be permitted to carry out reduced enrichment under close IAEA monitoring and Western sanctions that have crippled its economy would be lifted.