Two Indian-crewed merchant ships carrying Australian coal have been stuck at Chinese ports for several months, union officials said Wednesday, with the sailors becoming the latest casualties of escalating tensions between Canberra and Beijing.

Australia-China tensions had been deteriorating in recent years and worsened further when Canberra supported calls for an independent inquiry into the origins of the coronavirus pandemic.

In response, an upset Beijing has reportedly imposed trade embargoes on Australia, including on coal -- a business worth some US$10 billion a year.

The Indian bulk carrier MV Jag Anand with 23 Indian crew and carrying more than 160,000 tonnes of Australian coking coal has been stuck at the Chinese port of Jingtang since mid-June after being refused permission to offload, said Abdulgani Serang, secretary of the National Union of Seafarers of India.

Another vessel, Anastasia, also carrying coal from Australia and with 18 Indians on board, has been at China's Caofeidian anchorage since August 3, he added.

"We have a humanitarian crisis at hand. Our seafarers are being treated inhumanely," Serang told AFP.

"The ships have become floating prisons and it is taking a heavy toll on the mental and physical wellbeing of the crew."

Serang said both ships have not been allowed to unload their cargo or depart for another port.

His union has written to the Indian and Chinese governments as well as the International Maritime Organisation (IMO), pleading for an end to the crisis.

"The seafarers are just doing their job and paying a heavy price for no fault of theirs," Serang said.

Coal is among the Australian exports reportedly targeted by China as tensions escalate between the two countries
Coal is among the Australian exports reportedly targeted by China as tensions escalate between the two countries AFP / WILLIAM WEST

"It is like a food delivery guy being hammered in a fight between neighbours."

A letter from the wife of one of the crew members, posted Friday on Twitter tagging Indian Prime Minister Narendra Modi's account, said the "deteriorating health of crew members is also a major concern".

The Great Eastern Shipping Company, which owns the Indian vessel Jag Anand, said it had offered to send the ship to Japan at its own cost but to no avail.

"Regrettably, none of our efforts have yielded results so far," a spokesman told The Hindu newspaper.

But China's foreign ministry said Wednesday in response to an AFP query about the Jag Anand that "China has never restricted it from leaving".

"The freight side did not wish to adjust the ship's transport arrangements, citing commercial rights and interests. This is the real reason behind the circumstances."

India-China ties have also worsened significantly in recent months.

In mid-June, 20 Indian soldiers were killed in a brutal, high-altitude border clash. China has not revealed how many casualties it suffered.

The nuclear-armed neighbours later sent tens of thousands of troops to the disputed frontier in the Ladakh region in the Himalayas.