TOKYO — Japan said Tuesday that it had extracted gas from offshore deposits of methane hydrate — sometimes called “flammable ice” — a breakthrough that officials and experts said could be a step toward tapping a promising but still little-understood energy source.

The gas, whose extraction from the undersea hydrate reservoir was thought to be a world first, could provide an alternative source of energy to known oil and gas reserves. That could be crucial especially for Japan, which is the world’s biggest importer of liquefied natural gas and is engaged in a public debate about whether to resume the country’s heavy reliance on nuclear power.

Experts estimate that the carbon found in gas hydrates worldwide totals at least twice the amount of carbon in all of the earth’s other fossil fuels, making it a potential game-changer for energy-poor countries like Japan. Researchers had already successfully extracted gas from onshore methane hydrate reservoirs, but not from beneath the seabed, where much of the world’s deposits are thought to lie.

The exact properties of undersea hydrates and how they might affect the environment are still poorly understood, given that methane is a greenhouse gas. Japan has invested hundreds of millions of dollars since the early 2000s to explore offshore methane hydrate reserves in both the Pacific and the Sea of Japan.