VIENNA — Big-power foreign ministers are joining Iran nuclear talks on a diplomatic rescue mission. But even their muscle is seen as unlikely to bridge differences on Tehran's atomic activities in time to meet the July 20 target date for a deal."Obviously both sides have set out positions that are irreconcilable," says Gary Samore, who left the U.S. team negotiating with Iran last year. "That's why this negotiation is not going to end in agreement."French Foreign Minister Laurent Fabius alluded to the impasse this week, saying "none of the big points" had been settled. He said "the near totality" of a draft agreement that is being laboriously worked on consists of blanks. And in comments published Thursday by Austria's Wiener Zeitung, British Foreign Secretary William Hague said that agreement was far from certain.Two diplomats who are familiar with the confidential talks said Thursday that U.S. Secretary of State John Kerry and the top diplomats from Britain, France and Germany were tentatively scheduled to arrive in Vienna starting Saturday. They said Russia's foreign minister and a high foreign ministry official from China could also fly in.Deep divisions persist over uranium enrichment, which can produce both reactor fuel or fissile warhead cores. The U.S. wants deep cuts in the program. Instead, Tehran has gone public with demands that it be allowed to hugely expand it.Iran insists it does not want nuclear arms. The big powers fear that its array of more than 9,000 centrifuges enriching uranium and about 10,000 on standby already gives it the ability to produce enough weapons-grade uranium for one weapon in three or four months. They argue that — with Russia providing fuel for its power reactor and ready to do so for future ones — it has no need for so many machines.But Tehran wants even more. Iran nuclear chief Ali Akbar Salehi said this week that his country wants to expand the output of its enrichment program over the next eight years to a level that would need about 190,000 current centrifuges. Salehi said Iran wanted to use 8,000 advanced models to achieve that goal.The U.S. is ready to accept only a fraction of that number. And it wants the enrichment program frozen at that low level for decades.The two sides have the option of extending their negotiations for six months past July 20 date by mutual content and as that date approaches, diplomats say negotiators will likely opt for at least some extra time.But Samore, who is now with Harvard's Belfer Center, thinks the sides may agree to the full six months, saying "there is no reason to believe that the fundamental disagreement ... can be resolved any time soon."Five big-power foreign ministers and a deputy foreign minister already joined the final stage of talks in November that led to a preliminary deal freezing Tehran's nuclear program in exchange for some sanctions relief. The present talks aim for a deal that lifts all sanctions in exchange for long-term nuclear caps.Samore argued that an extension of negotiations while keeping the status quo was in the interest of both Iran and the United States."From the U.S. standpoint, the interim agreement has been a success in freezing the (Iranian nuclear) program by and large without allowing the sanctions regime to collapse," he said. Iran, in turn, has "gotten respite from additional sanctions pressure.""Right now, both sides would prefer an extension of the truce."