"It is possible that somewhere at the reactor may have been damaged," said Hidehiko Nishiyama, a spokesman for the nuclear safety agency. But he added that "our data suggest the reactor retains certain containment functions," implying that the damage may have occurred in Unit 3's reactor core but that it was limited.

Officials say the damage could instead have happened in other equipment, including piping or the spent fuel pool.

Operators have been struggling to keep cool water around radioactive fuel rods in the reactor's core after the March 11 earthquake and tsunami cut off power supply to the plant and its cooling system.

Damage could have been done to the core when a March 14 hydrogen explosion blew apart Unit 3's outer containment building.

This reactor, perhaps the most troubled at the six-unit site, holds 170 tons of radioactive fuel in its core. Previous radioactive emissions have come from intentional efforts to vent small amounts of steam through valves to prevent the core from bursting. However, releases from a breach could allow uncontrolled quantities of radioactive contaminants to escape into the surrounding ground or air.

Operators stopped work Friday at units 1 through 3 to check on radiation levels.