TOKYO — Japan took a major step back on Friday from earlier pledges to slash its greenhouse gas emissions, saying a shutdown of its nuclear power plants in the wake of the Fukushima disaster had made previous targets unattainable. The announcement cast a shadow over international talks underway in Warsaw aimed at fashioning a new global pact to address the threats of a changing climate.

Under its new goal, Japan, one of the world’s top polluters, would still seek to reduce its current emissions. But it would release 3 percent more greenhouse gases in 2020 than it did in 1990, rather than the 6 percent cut it originally promised or the 25 percent reduction it promised two years before the 2011 nuclear disaster.

The new target, however, is based on Japan’s never turning on any of its scores of idled plants, despite the government’s continued push to bring many of them back online. Highlighting the increased emissions that would result from staying nuclear free — and perhaps securing the backing of climate-change advocates — could allow the conservative government to strengthen the case to its own public that nuclear power is necessary.

Japan’s environment minister, Nobuteru Ishihara, said that the new target “does not consider the possible effect of nuclear power plants reducing emissions” and that Japan “would set a more definite target” once it settled on what sources of energy it would use in the future.