The goal is to rid the city of Taliban forces before the Muslim holy month of Ramadan begins in August, according to the official.
U.S. officials have previously disclosed plans for a NATO-led offensive in the area this year, but have not said when. The two-month offensive will be a major test of President Barack Obama's new strategy in Afghanistan and a bellwether of the war in general.
The official, who was not authorized to speak publicly on the operation, discussed it on condition of anonymity.
Until the start of major military operations, U.S. troops are working on securing transit routes and persuading the leaders of districts surrounding Kandahar to cooperate with NATO forces.
As the Taliban's governing capital before the U.S.-led invasion in 2001, Kandahar remains the spiritual heartland of the insurgency and a stubborn holdout in NATO's efforts to transfer control to Afghan President Hamid Karzai.
In an effort to reverse gains made by the Taliban, Obama has ordered the deployment of 30,000 more troops to Afghanistan. Military officials say they expect "several thousand" of those troops to be sent to Kandahar, mostly to partner with local police and provide a security presence in the region.
Gen. Stanley McChrystal, the top commander in Afghanistan, said earlier this month that the operation in Kandahar will be different from Marjah, where U.S. and Afghan forces stormed the farming town to purge Taliban forces. Unlike Marjah, the Kandahar operation will be a rolling series of actions and won't open with a single major offensive, McChrystal said.