TOKYO — Japan’s nuclear crisis intensified again Wednesday, with Japanese authorities announcing that a containment vessel in a second reactor unit at the stricken Fukushima Daiichi plant in northeastern Japan may have ruptured and appeared to be releasing radioactive steam. That would be the second vessel to be compromised in two days.

The vessel had appeared to be the last fully intact line of defense against large-scale releases of radioactive materials from that reactor, but it was not clear how serious the possible breach might be.

The announcement came after Japanese broadcasters showed live footage of thick plumes of steam rising above the plant.

Yukio Edano, the chief cabinet secretary, said the government believed the steam was coming from the No. 3 reactor, where an explosion on Monday blew out part of the building surrounding the containment vessel.