A sudden acceleration in India's nuclear weapons program has heightened U.S. concern over an eventual atomic conflict between the Asian nation and its neighboring adversary Pakistan, administration sources say. U.S. intelligence indicates India has stepped up production of nuclear arms to as many as 20 a year and is trying to acquire materials that would increase the kill power of warheads, the sources have told United Press International. In a report last month, UPI quoted intelligence sources and congressional staffers as saying India already had prepared a few sophisticated, low-yield atomic bombs that could be used with combat aircraft.