QINGHUANGDAO, Hebei: Emergency services carried out their largest-ever drill yesterday to tackle an oil spill in the Bohai Sea, putting to the test their skills at protecting the ocean environment.

The exercise, which started at 9:30 am, involved a scenario in which a 10,000-ton oil tanker exploded and spilt 500 tons of oil into the sea.

At least 20 vessels, two helicopters and more than 500 people including 300 volunteers participated in the exercise.

The maritime safety administration of China, which organized the drill, said the risk of an oil spill happening in China's sea areas was higher than ever and a constant threat to the ocean environment.

More than 90 percent of China's oil imports - the country is the world's second largest oil importer - is transported by sea. Some 160,000 tankers of all sizes sailed into and out of China's ports last year, an average of more than 400 every day.

Bohai Bay, the mouths of the Yangtze and Pearl rivers, and the Taiwan Straits are the four areas most at risk of an oil spill, the administration said.

Bohai Bay is especially at risk because it has almost 100 ports. Three of them - Dalian, Qinhuangdao and Tianjin - each handle more than 100 million tons of oil every year.

With the recent discovery of a new oilfield in the bay with estimated deposits of 1 billion tons, maritime officials said the number of vessels carrying oil, chemicals and other dangerous materials will climb rapidly, making the already tricky navigation conditions even more complicated.

Liu Gongcheng, director of the maritime safety administration, said at a press conference yesterday: "Major ports in coastal areas are capable of dealing with oil spills caused by small- and medium-sized vessels in waters close to the shore."

The emergency system used in major harbors, which absorbs spilt oil, can handle up to 200 tons of oil leaked from vessels into the sea, Liu said.

However, he said that more money will need to be made available to allow for the purchase of more oil-absorbing materials to cope with bigger spills.

Liu said the administration will continue to invest in the sector, and aims to catch up with middle-level developed countries by 2010, which have the ability to handle spills of up to 500 tons.

Between 1973 and 2006, 2,635 oil spills occurred in China's coastal waters, involving 37,000 tons of oil. Nearly 70 of the accidents were considered serious, involving leaks of more than 50 tons each.

The most serious occurred in 1983 when a Panama-flagged ship leaked more than 3,300 tons of oil into the sea near Qingdao in East China's Shandong Province. The oil spill polluted 230 km of shoreline.