Iran nuclear talks yield no breakthrough MOSCOW - Iran and six world powers negotiating over Tehran's disputed nuclear program adjourned Tuesday after a contentious two-day session without scheduling a high-level follow-up meeting, leaving the future of the talks in doubt. After failing to narrow their differences in five marathon meetings, the group scheduled a meeting of lower-level experts next month in Istanbul, Turkey, that will focus on technical details of both sides' proposals. The major powers might propose a resumption of high-level talks later if justified. In the meantime, the uncertain outlook could sharpen tensions in the oil markets and heighten war worries. Israeli officials have warned that they might bomb Iran to end its nuclear program if the diplomatic effort faltered. A senior Obama administration official, asked about the likelihood of a resumption of top-level talks, said that "these decisions have not been taken yet because we have to see how things evolve." "We're taking this step by step and we are going to see if Iran is prepared to make the choices it needs to make," said the official, who requested anonymity, as is customary at such negotiations. The six countries - Britain, France, Germany, Russia, China and the United States - have been trying to ease into a negotiation aimed at putting curbs on Iran's nuclear program, which many nations fear is aimed at developing a nuclear weapons capability. The major powers have badly wanted to keep the talks going to minimize the risk of a conflict that could drive up oil prices and upend the world economy. But in three separate meetings, starting in April, the two sides have remained far apart. That was underscored Monday when the chief Iranian negotiator, Saeed Jalili, lambasted an interim negotiating proposal put forward by the six powers. "It remains clear that there are significant gaps between the substance of the two positions," the European Union's foreign policy chief, Catherine Ashton, said at a news conference. Although the six powers have had an interest in keeping the talks going, they have also been under intense political pressure at home and from Israel not to allow the talks to continue without progress. Critics note that Iran continues to enrich uranium that it could potentially use to develop a bomb-making capability. Last week, Israeli President Shimon Peres complained that the talks were not making sufficient progress and 44 U.S. senators wrote President Obama urging that he reconsider the talks if the Moscow meeting produced no tangible progress. The six powers are hoping that pressure on Iran will build at the end of this month, when a new round of European and American sanctions will take effect, further damaging its reeling economy. "The pressure on Iran will be increasing," the senior U.S. official said. But current and former U.S. officials acknowledge that it is still unclear whether Iranian supreme leader Ayatollah Ali Khamenei has decided he is willing to have his country endure a badly damaged economy in return for a nuclear weapons capability that would safeguard the regime's survival. He may be willing to let Iranians "eat bark" to gain a nuclear capability, said Ray Takeyh of the Council on Foreign Relations, a former administration advisor on Iran. U.S. officials have been seeking an agreement under which Iran would agree to halt production of uranium enriched to 20%, close down an underground facility where it is being made, and ship the current stockpile out of the country. Uranium enriched to 20% can be modified with relative ease into a form usable in nuclear bombs. Iran has been pushing the six countries to recognize what it sees as its right to enrich uranium and to ease sanctions that have undermined its currency, driven up prices and isolated its economy from the rest of the world. But the senior Obama administration official said the six powers had not sweetened their offer at all during the course of the two-day meetings. Jalili, the Iranian negotiator, told reporters that the talks in Moscow were "more serious, more realistic," but also acknowledged they could have been more successful. He reiterated that Iran would not yield on its right to enrich uranium. Sen. Mark Steven Kirk (R-Ill.), one of the foremost advocates of sanctions on Iran, said in a statement that after the three rounds of meetings Iran remains in violation of U.N. Security Council resolutions calling on it to halt uranium enrichment. He called on Congress to finish work on a new set of sanctions targeting Iran's financial, energy, shipping and insurance businesses, and to send it to Obama next month.