China claims 'quality' problem with Australian coal as $700 million worth sits idle off ports

China has suggested almost $700 million worth of Australian coal is being held up at ports due to "environmental quality" problems.

Key points:

It is the first time Chinese authorities have suggested a reason for the delays to unload ships

Coal is one of seven Australian imported products targeted with bans by China

But China denies it is levying coordinated trade action against Australia

For months, dozens of bulk carriers have been stranded off the coast of two major Chinese ports unable to unload their cargoes, with a Bloomberg estimate of more than 60 ships now in limbo in November.

Chinese authorities have not previously explained the exact reasons for the long delays, which have coincided with a series of restrictions and bans Beijing has imposed on other Australian exports amid diplomatic tensions.

But in answer to a question on Tuesday, China's Foreign Ministry spokesman Zhao Lijian has for the first time suggested quality problems are to blame.

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"In recent years, China Customs has conducted risk monitoring and analysis on the safety and quality of imported coal and discovered imported coal not meeting environmental standards is relatively common," he said.

China has unofficially banned Australian coal imports since October amid souring relations between the two countries, and in turn, increased imports from Mongolia and Russia.

Mr Zhao said China had strengthened the examination and testing of imported coal regarding safety, quality and environmental standards "so as to better protect the legitimate interests and the environmental interests of the Chinese side".

Coal is one of seven Australian imported products that have reportedly been targeted with bans by China amid rising tensions.