`More than 5,500 illegal immigrants have been prevented from entering Bulgaria since the beginning of this year,` the Director of the National Border Police Service, Colonel Valeri Grigorov, told journalists in Vidin. He specified that the situation on Bulgaria`s border with Turkey turns out to be the most serious problem to the Bulgarian Border Service. This border, being a future external frontier of the European Union, will be among the most closely guarded and watched ones. Thanks to tightened security, Bulgarian border policemen stopped 1,877 persons trying to cross the green border from Turkey between the beginning of January and the end of September. `More than 2,600 people who did not hold valid travel documents have been turned back from the checkpoints of Malko Turnovo and Kapitan Andreevo (on the border with Turkey),` Colonel Grigorov said. Six regional border sector chiefs and seven border checkpoint chiefs were replaced in 2000 because of bad performance. Out of 16 chiefs dismissed last year, 12 have been referred to the prosecuting authorities and one has been convicted. Colonel Grigorov singled out the state-of-the art marking as a special success in the defense of Bulgaria`s border with Yugoslavia. The marking uses special digital maps with GPS grid references. The map sheets are being prepared, and the border marking will be completed until the end of this year. It will give Bulgaria up-to-date demarcation of an international border. Colonel Grigorov was in Vidin for the introduction of Major Yordan Yankov, the new Chief of the Regional Border Sector.