China has stopped the Dalian oil spill from reaching international waters, an official said Monday, admitting the clean-up work was "arduous."

Dai Yulin, vice mayor of Dalian City, Liaoning Province, where oil pipelines exploded on July 16, said workers had contained the oil slick, stopping it from reaching the open sea.

"But the next step, which is clearing it up, is an arduous task," Dai told a press briefing.

"Some of the slick has been mopped up, but it's not easy to get rid of the rest," he said.

The cleanup has involved 266 oil-skimming vessels and 8,150 fishing boats, Dai told reporters.

Maritime agencies and oil companies have laid down more than 40,000 meters of oil barriers and 65 tonnes of oil absorbent mats, he said.

Despite this, oil could still be seen on some beaches.

An explosion hit an oil pipeline 0.9 meters in diameter at 6:20 pm on July 16 and triggered an adjacent smaller pipeline to explode near Dalian Xingang Port. Both pipelines are owned by China's No.1 oil and gas producer CNPC.

Improper injections of strongly oxidizing desulfurizer into the oil pipeline after a 300,000-tonne tanker had finished unloading its oil caused the explosion, results of a State Administration of Work Safety and Ministry of Public Security investigation showed Friday.