Glencore PLC and Vitol Group, the world’s two largest independent oil traders, are close to signing a long-term deal to purchase Iranian crude, a top Iranian official said. The potential deal would mark the return of two of Iran’s biggest oil trading partners before Western sanctions over the country’s nuclear program were tightened in 2012. Iran has sought to ramp up production since those sanctions ended in January. But many Western buyers have been reluctant to purchase its oil because their banks are concerned about falling foul of remaining U.S. sanctions. Glencore and Vitol representatives declined to comment. Iran and the two Swiss-based companies are close to reaching separate agreements with state-owned National Iranian Oil Co., said Mohsen Ghamsari, the company’s director of international affairs, in an interview.