TOKYO — Japan vowed Tuesday to stick to its goal of stabilizing a stricken nuclear power plant in as soon as six months, despite recently acknowledging that damage to the plant’s reactors might be worse than initially estimated.

The operator of the Fukushima Daiichi nuclear power plant, Tokyo Electric Power Company, has in the past week offered a graver account of how the accident unfolded after the facility was ravaged by the tsunami that followed the March 11 earthquake. The company now acknowledges that a fuel meltdown occurred at three of the plant’s six reactors in the early hours of the crisis, something experts had been suggesting for weeks.

“Even though we have confirmed that the nuclear core melted soon after the accident, our three- to six-month timeline will not change,” Tokyo Electric’s nuclear chief, Sakae Muto, said at a news conference on Tuesday.

Tokyo Electric said it would change some aspects of its plan to bring the reactors to what it called a “cold shutdown,” where temperatures at the core fall below the boiling point.