Japan's Environment Ministry says tritium in seawater off the Fukushima Daiichi nuclear power plant remains below the detectable level after the recent release of treated and diluted water.

Ministry officials have been conducting a weekly survey of seawater samples around the plant since Tokyo Electric Power Company began the discharge on August 24. The first round of the water release was completed on September 11.

In the fourth survey, samples were collected for three days through last Friday from 11 points off Fukushima Prefecture. This included a spot near the water discharge outlet, while the furthest point was some 40 kilometers away.

The Environment Ministry says the concentration of tritium at all 11 points was below the minimum detectable level of 10 becquerels per liter.

The tritium was also below the detectable level in the results that were released on August 27, September 1 and September 11.

The ministry will continue monitoring tritium levels once a week. The results will be published on its website and social media.

TEPCO plans to carry out three more rounds of the water discharge by the end of next March. It plans to start the second round when the preparations are completed.

The Fukushima Daiichi plant suffered a triple meltdown in the 2011 earthquake and tsunami.

Water used to cool molten fuel has been mixing with rain and groundwater. The accumulated water is being treated to remove most radioactive substances, but still contains tritium.

Before releasing the treated water into the sea, the plant's operator dilutes it to reduce tritium levels to about one-seventh of the World Health Organization's guidelines for drinking water.