Japan’s nuclear crisis intensified on Wednesday after the authorities announced that a second reactor unit at the stricken Fukushima Daiichi plant in northeastern Japan may have ruptured and appeared to be releasing radioactive steam.

The break, at the No. 3 reactor unit, worsened the already perilous conditions at the plant, a day after officials said the containment vessel in the No. 2 reactor had also cracked.

The possibility of high radiation levels above the plant prompted the Japanese military to put off a highly unusual plan to dump water from helicopters — a tactic normally used to combat forest fires — to lower temperatures in a pool containing spent fuel rods that was overheating dangerously at the No. 4 reactor. The operation would have meant flying a helicopter into the steam rising from the plant.