Chinese authorities on Friday blamed a chemical used to remove sulphur from crude oil for a blast at a storage facility that caused a spill on the country's northeast coast.

A worker cleans up at the oil spill site near Dalian port in Liaoning province on Friday. The Xingang oil port has resumed some refined fuel loading for the domestic market but fuel exports remain temporarily halted, industry officials said amid the continuing cleanup operation. [Reuters]

An investigation has found that a desulphurising chemical was mistakenly pumped into pipelines after a tanker had stopped unloading crude at the port city of Dalian last Friday, triggering the explosion, the State Administration of Work Safety said in a statement posted on its website.

The 0.9-meter-diameter oil pipeline exploded at 6 pm on July 16, triggering a smaller adjacent pipeline to also explode, the statement said.

The explosion occurred as workers from the Shanghai-based QPRO Inspection and Technical Service. continued to inject desulfurizer into the pipeline after the 300,000-ton tanker had finished unloading its oil at 1 pm.

Produced by the Tianjin-based Huishengda Petroleum Technology, the desulfurizer was strongly oxidizing, according to the statement.

A subsidiary of the China National Petroleum Corporation, Asia's biggest oil and gas producer by volume, had authorized the two companies to conduct the operation.

The cause of China's largest reported oil spill was announced as environmentalists urged the government to do more to warn local residents of the potential danger, saying children are still playing on nearby beaches.

Chinese authorities gave no update on Friday on the size of the oil spill, which had spread over at least 435 square kilometers of water after the blast.

The government has mobilized hundreds of fishing boats and other vessels to clean up the spill.

The disaster has caused China to take a hard look at its ports, which are some of the busiest in the world.

Officials warned of a "severe threat" to sea life and water quality as China's latest environmental crisis spread off the shores of Dalian, once named China's most livable city. One cleanup worker drowned this week, his body coated in crude.

Workers reported using chopsticks and their bare hands to remove the gooey oil from the sea, while State media said 2,000 soldiers, 40 oil-skimming boats and hundreds of fishing boats were helping with the cleanup.

Environmental group Greenpeace, which has a team at the scene, urged the government to warn residents on nearby coastlines of the dangers.

"Greenpeace was surprised to see that the beaches have not been closed to visitors and lack any warning signs," Greenpeace China said in a statement on Friday evening. "As a result, locals and visitors, unaware of the extent of the oil spill, were playing in the water with their kids, risking exposure to petroleum."

The statement said fishermen without equipment were doing most of the cleanup work at one of Dalian's most popular beaches, Jinshitan.

"They don't even have face masks, the most basic and necessary of precautions. They don't even know that they need to protect their skin from crude oil," said Zhong Yu, one of the Greenpeace workers.

"We strongly urge the government to send professional staff and safety equipment for the cleanup process," Zhong said in the group's statement.

China Central Television reported that about 1,500 tons of oil has spilled, roughly amounting to 400,000 gallons, as compared with 94 million to 184 million gallons in the BP oil spill off the United States' coast.