HELSINKI (AFP) - Illegal oil dumping from ships into the Baltic Sea has decreased significantly over the past decade despite increased traffic as surveillance has improved, an intergovernmental authority said Friday.

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"This is a very excellent development," said Anne Christine Brusendorff, executive secretary of HELCOM, a marine environment protection organisation made up of the European Union and the nine countries surrounding the Baltic Sea.

Last year, HELCOM detected 178 deliberate, illegal oil spills from ships, a drop of 15 percent from 2008 and down by a quarter compared to 2007.

Illegal dumping has fallen by more than 60 percent since 1999, when 488 cases were detected by national surveillance planes, and the size of spills has also fallen, HELCOM said in a statement.

But the culprits of illegal oil dumping often go unpunished as the polluters remain unknown "in the vast majority of cases", HELCOM said. Last year, for example, the polluters were only identified in eight of the detected cases.

HELCOM countries -- Russia, Finland, Sweden, Denmark, Germany, Poland, Lithuania, Latvia and Estonia -- have conducted regular surveillance flights since 1988 and carried out measures to prevent illegal oil and waste discharges into the sea since the 1990s.

The peak year was 1989, when 763 illegal spills were detected, said HELCOM, whose observation fleet now consists of more than 25 planes and helicopters.

It also uses satellite surveillance to detect environmental crimes and their potential perpetrators.

Over-fished and affected by agricultural nutrient discgarge and industrial waste, the Baltic is one of the most polluted seas in the world, and Greenpeace says it is so toxic pregnant women should not eat its fish.

At a summit earlier this year, leaders from the Baltic Sea region pledged urgent action to save the ailing body of water.