A senior U.S. official says the U.S. has begun airstrikes in Tikrit in support of a stalled Iraqi ground offensive to retake the city from Islamic State of Iraq and al-Sham(ISIS) fighters.The official says the airstrikes began after the Iraqi government requested U.S. help. The official spoke on condition of anonymity because the American attacks had not yet been officially announced.An Associated Press reporter in Tikrit reported hearing warplanes overhead late Wednesday, followed by multiple explosions.