Iran and six world powers have agreed to a four-month extension of negotiations on a nuclear deal with Tehran after failing to meet a July 20 deadline due to "significant gaps" between the two sides, the European Union and Iran said on Saturday."There are still significant gaps on some core issues which will require more time and effort," EU foreign policy chief Catherine Ashton and Iranian Foreign Minister Mohammad Javad Zarif said in a joint statement.Iran, the United States, Britain, France, Germany, Russia and China had set a July 20 deadline to complete a long-term agreement that would resolve the decade-old dispute over Tehran's nuclear ambitions. But diplomats said they were unable to overcome significant differences on major sticking points."We will reconvene in the coming weeks in different formats with the clear determination to reach agreement on a Joint Comprehensive Plan of Action (long-term agreement) at the earliest possible moment," Ashton and Zarif said.Ashton was speaking on behalf of the six powers. The extension begins July 21 and runs through November 24.The White House said on Friday there was a "credible prospect for a comprehensive deal" with Iran over its nuclear program that made it necessary to extend talks with Western powers another four months."This extension will allow us to continue the negotiations while ensuring that the progress of Iran's nuclear program remains halted during the negotiations," White House spokesman Josh Earnest said in a statement."We have an opportunity to achieve a lasting, diplomatic solution that will resolve one of the most pressing national security issues of our time. We will not accept anything less than a comprehensive resolution that meets our objectives, which is why it is necessary for negotiations to continue." US Secretary of State John Kerry released a statement on Saturday, following the agreement. "As President Obama and our entire administration has made clear, we are committed to testing whether we can address one of the world’s most pressing priorities – ensuring that Iran does not obtain a nuclear weapon – through the diplomatic negotiations in which we and our international partners are currently engaged," it read. He added that the effort to reach an agreement was "as intense as it is important," adding that the US has come a long way in a short period of timeIran will be allowed to access an additional $2.8 billion of its frozen assets during a 4-month period of extended talks with six powers on its nuclear program but most sanctions against Tehran will remain in place, the United States said on Saturday."We will continue to suspend the sanctions we agreed to under the preliminary agreement from November 2013 and will allow Iran access to $2.8 billion dollars of its restricted assets," Kerry said in response to the agreement. "Let me be clear," Kerry said in a statement. "Iran will not get any more money during these four months than it did during the last six months, and the vast majority of its frozen oil revenues will remain inaccessible."This comes after Kerry was in Vienna for talks with Iran to see if the Islamic Republic was truly prepared to make the concessions necessary for such a deal, the White House said on Monday. After the direct talks on Monday morning with Iran, officials from the European Union joined discussions in the afternoon.A formal line of communication between the US and Iran first opened last September at the United Nations General Assembly, after 34 years of frozen diplomatic relations. Talks before then between Washington and Tehran were rare, and private.First and foremost among the gaps facing negotiators are expectations over Iran's uranium enrichment capabilities. A major address delivered last week by Ayatollah Ali Khamenei, Iran's supreme leader, called for a marked increase in Iran's enrichment capacity, after world powers made clear for months they would settle for nothing less than a significant reduction of Iran's centrifuge infrastructure.