Four new fires broke out Friday in the devastation left by giant explosions in the Chinese port city of Tianjin last week, state media reported, as clean-up efforts continued.

New fires have repeatedly broken out on the site, which is scattered with smouldering chemicals and flammable substances, the official Xinhua news agency said. Pictures of the area have regularly shown smoke rising from various points.

One fresh blaze was in the depot where at least 3,000 cars were torched, and suspected to be caused by fuel in vehicle tanks, it said. There were fires at three other locations in the "central blast zone", it added.

The explosions at a hazardous goods storage facility last week triggered a giant fireball and killed at least 116 people, officials said, with 60 others missing and hundreds more injured.

The blasts have also sparked fears of toxic pollutants contaminating the air and water of the city, which has a population of around 15 million people.

Thousands of tonnes of hazardous chemicals were stored at the site, officials have said, including about 700 tonnes of highly poisonous sodium cyanide, a white powder or crystal which can give off lethal hydrogen cyanide gas.

Officials have sought to reassure the public, but cyanide levels 356 times permitted limits have been found in water at one test point in the site, and the Tianjin environmental protection bureau said on its website cyanide five times permitted limits was detected at an air test point on Friday before falling.

Tianjin's vice mayor Wang Hongjiang told a press conference that 200 tonnes of sodium cyanide had been collected so far.

Greenpeace said Friday it had detected low levels of cyanide in surface water at three test points in Tianjin, two of them near the site and one at the Haihe river, where pictures on social media had shown thousands of dead fish.

Tianjin officials have said that fish frequently die en masse in local rivers during summer, due to the poor quality of the water, and that they had not detected toxic levels of the chemical.

More than 4,460 soldiers and armed police officers have been deployed to collect the substances and clean the area, said the search and cleanup operation's top military commander Liao Keduo, according to Xinhua.