Reports of multibillion-dollar oil talks between Iran and Russia are emerging as the latest obstacle to a comprehensive pact eliminating the threat of an Iranian nuclear arsenal. The Obama administration is weighing potentially deal-breaking sanctions if a contract is completed. The Russian business daily Kommersant has reported Russia plans to buy 500,000 barrels of Iranian oil a day, shattering an export limit under the interim nuclear agreement world powers and Iran reached last year. Moscow and Tehran are far from finalizing the contract, the newspaper said, but the U.S. has expressed alarm. A senior U.S. official said the administration had no information to suggest the oil-for-goods contract had been reached. The arrangement would break the interim agreement reached in November in Geneva and “potentially trigger U.S. sanctions,” said the official, who was not authorized to be quoted by name and briefed reporters on condition of anonymity. […]