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

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 said. The spill was contained by an oil fence, a TEPCO spokesman said.

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

The main problems at Fukushima Dai-ichi 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.