Estonian officials have narrowed down their search for the ship that caused a 35-kilometer (21-mile) oil slick that has killed at least 800 birds in the Baltic Sea, singling out a Liberian-flagged tanker as the possible culprit.

"The oil tanker Flawless, which was on its way from Muuga port, near Tallinn, to Singapore, may be the tanker that polluted the Estonian coastline," Prime Minister Andrus Ansip told a press conference.

Some 5,000 birds are feared to have died and more than 800 cadavers have already been recovered by environmentalists and volunteers since the long oil slick was first noticed off the coast of the Baltic state on Saturday.

Ansip said the Flawless had reported last week to the coastguard that more than 1.5 tonnes of oil had been accidentally pumped to the upper deck of the ship on January 23 when it was loaded with its petroleum cargo.

According to the prime minister, the coastguard passed the information to Environment Ministry inspectors, but the ministry did not take the matter any further.

Environment Minister Villu Reiljan said he had heard nothing about any problems with the Flawless before the prime minister made public the information on Thursday.

Reiljan said that eventual leaks from other oil tankers which passed off Estonia's northwestern coast near the scene of the environmental disaster were also being investigated as the possible cause of the slick.