TOKYO — An oil spill and a small explosion have caused limited damage — but no further radiation leaks — at the crippled nuclear power plant in northeastern Japan, the plant operator said Tuesday.

Workers at Fukushima Dai-ichi plant found an oil spill in the sea near reactors five and six, which were in shutdown when the earthquake and tsunami struck March 11, Tokyo Electric Power Co. said. The spill was contained by an oil fence, TEPCO spokesman Taichi Okazaki said.

The explosion workers heard at reactor four was likely from a gas tank and did not cause any additional radiation leaks, Okazaki said. The cause was being investigated.

The main problems at Fukushima Dai-ichi are involve reactors one, two and three, where the fuel cores have melted. Scientists and government officials say the reactors are short of a full meltdown, in which the fuel breaks through the bottom of the container.

Workers have been fighting to get the reactors under control after the tsunami destroyed backup power generators, halting crucial cooling systems that managed the fuel temperature.

In the immediate days after the tsunami, several explosions larger than Tuesday’s hit the plant and scattered highly radioactive debris and puffs of radioactivity particles into the environment. The plant has also leaked tons of radioactive water, which officials are promising to clean up.

TEPCO has promised to bring the plant under control by January, but fears are growing that was too optimistic.