The head of the Syrian Petroleum Company, Youssef Qeblawi, said on Monday that oil major Shell had asked to withdraw from the al-Omar oilfield and transfer its share to Syria's state-owned operators but that U.S. companies were interested in the country's energy sector.The al-Omar oilfield, Syria's largest, came under government control at the weekend after a lightning offensive against Kurdish forces who had held the site for nearly a decade and used it as a military base…