A Bulgarian police officer has been shot dead by an unidentified person at the border with Turkey, officials say.
The incident occurred on Monday evening when the border police officer and a soldier were patrolling near the village of Golyam Dervent, according to interior minister Ivan Demerdzhiev.
The two officers had allegedly found damage on the barbed wire border fence before "shots rang out".
The 30-year-old policeman died at the scene while the serviceman was uninjured.
Bulgarian authorities believe the gunman fired their weapon from Turkish territory and have dispatched additional border patrols.
Demerdzhiev told reporters that he had received assurances from Turkey that the shooter will be captured and "brought to justice".
"This is ... the first gun attack in years," he added. "The patrols are facing an unprecedented refugee surge every day, last night’s case was an extreme act of aggression."
Demerdzhiev has also called for more support from Europol and the European Commission.
Bulgaria has deployed 350 troops and 40 army vehicles along its 259-kilometre southern border with Turkey amid a growing migrant influx.
Bulgaria had already stepped up measures after two policemen were killed by a bus carrying migrants in late August.
According to official figures, around 12,700 illegal immigrants have been detained in the country since the start of 2022 -- 2.5 times more than the same period last year.
Most migrants arriving in Bulgaria from the Middle East use the country as a transit corridor to western Europe.