Six days after a fire broke out on an Iranian oil tanker off the coast of China, firefighters are still struggling to extinguish the blaze, as criticism mounted in Tehran over whether enough was being done to locate dozens of its missing crew members.

The Sanchi, carrying 136,000 tonnes of light crude oil from Iran to South Korea, has been in flames since colliding with the CF Crystal, a Hong Kong-registered bulk freighter, 160 nautical miles east of Shanghai on Saturday.

One body has been found but 31 sailors – mainly Iranians – remain missing with officials in Iran hoping they have found sanctuary on an unaffected part of the vessel.

Handout picture taken on 10 January and released by the transport ministry of China. Photograph: -/AFP/Getty Images

Cleanup and rescue ships have faced toxic fumes, strong winds, rain and waves up to four metres high as they scrambled to find survivors and avoid a massive oil slick since Saturday’s incident.

China’s transport ministry said on Thursday evening the Panamanian-flagged 274-metre (899ft) tanker remained on fire, adding two ships had spent the day spraying the vessel with retardant foam.

But rescue efforts were still being hampered by “terrible” weather conditions and toxic gases from the burning oil which “pose a great danger to rescuers”, the ministry said in a statement.

Of the 30 Iranians and two Bangladeshis on the Sanchi’s crew, only one body has so far been found and there has been no word from them since the collision.

Photo provided by the Korea coast guard from 7 January 2018. Photograph: AP

On Thursday the Iranian Merchant Mariners Syndicate, an industry NGO, said it had written to Chinese authorities expressing frustration at the lack of progress in putting out the fire.

“It was clear that the Chinese are not cooperating enough,” IMMS vice-president Saman Rezaie, told AFP.

Iranian news website Khabar Online published video interviews with some of the relatives of those missing.

“I don’t know what else to say but just beg them to rescue them as I am sure that they are alive inside,” said one woman, who identified herself as the wife of the Sanchi’s chief engineer.

The 21 Chinese crew members of the Crystal, which did not burst into flames, were all rescued.