TOKYO — Radiation levels have risen inside a reactor unit at a nuclear power plant in western Japan, possibly due to leaks from fuel rods into cooling water, the facility’s operator said on Monday.

No radiation was released outside, the Japan Atomic Power Co. said in a press release.

It occurred at a plant in Tsuruga, some 350 kilometres (220 miles) west of Tokyo and 90 kilometres from Kyoto, while emergency crews continue efforts to stabilise the tsunami-hit Fukushima nuclear plant in northeast Japan.

According to a regular check at Tsuruga on Monday, the density of xenon-133 gas and that of iodine-133 had risen inside the primary coolant water in one of the plant’s two nuclear reactors, the press release said.

“We have determined that there may have been leakage from the fuel assembly and stepped up monitoring the density of radioactive substances in the primary coolant,” it said.

The company was considering shutting down the reactor for detailed examinations, the statement said.