Japan's Prime Minister Naoto Kan reacts in front of the Japanese flag with a black mourning cloth during a news conference at his official residence in Tokyo March 25, 2011. Kan said on Friday that the situation at a quake-damaged nuclear plant in northeast Japan remained very precarious.[Photo/Agencies]

TOKYO - Japan's prime minister says the situation at the crippled Fukushima Dai-ichi nuclear power plant remains "grave and serious."

In a televised address Friday evening, Naoto Kan warned that "we are not in a position where we can be optimistic. We must treat every development with the utmost care."

He also apologized to farmers and business owners around the plant for damage caused.

Nuclear safety agency officials said Friday they suspected a breach in the reactor core of one unit at the plant. The complex was damaged by a March 11 earthquake and tsunami.

That could mean more radioactive contamination in the evironment.

The uncertain situation halted work at the plant, where dozens had been trying to stop the overheated plant from leaking dangerous radiation.