Bulgaria's interior ministry says the number of migrants illegally entering the country has more than trebled in 2021.
Bulgaria has sent 350 additional military personnel to its border with Turkey amid rising migration levels.
Defense Minister Georgi Panayotov told Bulgarian media that the troops were arriving to "support the border police".
"350 people with some 40 units of equipment have been sent," he said in an interview with bTV.
More than 6,500 migrants were detained after illegally entering Bulgaria from January to September, according to the Interior Ministry.
The number of entrants is more than three times as many as in the same period in 2020, the ministry said.
In August, the Bulgarian parliament also voted to send between 400 and 700 soldiers to help build fences along its borders with Greece and Turkey.