Tokyo Electric Power Co. has seen a rise in the level of radioactive tritium in seawater within the harbor at the crippled Fukushima No. 1 nuclear power plant.

A sample collected Friday contained around 1,100 becquerels of tritium per liter, the highest level detected in seawater since the nuclear crisis at the plant started in March 2011, the utility said Monday.

An official of the Nuclear Regulation Authority said groundwater containing radioactive substances may be seeping into the harbor from the plant site and there is a need to carry out a careful investigation because the data collected so far are limited.

According to Tepco, the sample with the highest tritium concentration was collected near a water intake on the east side of the reactor 1 turbine building. The level was more than double that of a sample taken on June 10 in the same area.

The latest announcement was made after Tepco detected high levels of radioactive tritium and strontium in groundwater from an observation well at the plant.

The government-set safety limit for tritium is 60,000 becquerels per liter of seawater.