VIENNA - World powers and Iran will not meet a self-imposed deadline for a comprehensive nuclear agreement scheduled for July 7, and will extend their interim agreement until July 10.Negotiations are at their most "difficult" and "real" phase, the European Union's foreign policy chief, Federica Mogherini, told press gathered outside the Palais Coburg in Austria's capital. The diplomatic effort has now been under way for two years."We are continuing to negotiate for the next couple of days. This does not mean we are extending our deadline," she said.US Secretary of State John Kerry's senior advisor for strategic communications, Marie Harf, said that negotiators are "frankly more concerned about the quality of the deal than we are about the clock." But they also are committed to concluding the process during this negotiating round, she added."We also know that difficult decisions won't get any easier with time," Harf said. "We are taking these negotiations day to day."Mogherini characterized the talks as "difficult" and "tense." All parties agree that the negotiations are in their final stage, if not in their final hours.Departing the Coburg, Russia's foreign minister, Sergey Lavrov, told Russian media that eight issues critical to a deal were still not finalized as negotiators entered their second deadline in seven days. Most foreign ministers would be leaving Vienna, but would return shortly, he said."There is only one big problem in terms of sanctions," Lavrov said. "It is the problem of a weapons embargo."The US, Britain, France, Russia, China and Germany had hoped to seal a comprehensive deal by June 30. Missing that deadline, they extended their effort to July 7. They seek to cap, restrict, monitor and partially roll back Iran's nuclear program in exchange for sanctions relief, after toiling with the prospect of a nuclear Iran for over a decade. Tehran wants that relief immediately.