China has renewed its opposition to the discharge of treated and diluted water from Japan's Fukushima Daiichi nuclear power plant into the ocean that began on Thursday.

Chinese Foreign Ministry spokesperson Wang Wenbin was asked about the government's responses to the release on Friday.

Wang did not answer the questions directly, but said the discharge is an extremely selfish and irresponsible act.

He urged Japan to refrain from acting arbitrarily and to dispose of what he called "nuclear contaminated water" in an appropriate and responsible way.

Asked whether Beijing intends to accept Japanese Prime Minister Kishida Fumio's call for discussions by experts based on scientific grounds, Wang again did not give a direct answer.

He said Japan should immediately rectify a selfish act that shifts the risk of contamination to the whole world.

Also on Friday, Tokyo Electric Power Company said levels of radioactive tritium in seawater samples taken near the plant are too low to be detected.

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

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

TEPCO dilutes the treated water to reduce tritium levels to about one-seventh of the World Health Organization's guidelines for drinking water quality before releasing it into the sea.