China on Wednesday expressed concerns over radiation monitoring in Japan's planned release of nuclear-contaminated water into the sea.

Due to a delay in pre-discharge monitoring, wastewater below safety standards may not be detected in time and may be released directly into the ocean, said an official with the Ministry of Ecology and Environment (National Nuclear Safety Administration).

Japan has mixed 10 tanks of contaminated water for sampling and monitoring. As a result, highly-concentrated wastewater may have been diluted in the process so that the monitoring results can meet discharge standards, the official said.

Instead of a monitoring conducted solely by Japan, there should be open and transparent monitoring and oversight by stakeholders from all over the world given that the discharge of Fukushima water is a global maritime environment and public health concern, according to the official.

The ministry has made arrangements for radiation monitoring once Japan begins releasing the nuclear-contaminated water, and will issue warnings if any abnormalities are detected to ensure and protect China's national interests as well as the health of Chinese people, the official added.

Japan plans to dump some 1.3 million tonnes of wastewater from its Fukushima Daiichi Nuclear Power Plant which suffered catastrophic damage during an earthquake in 2011.