A specialist bomb disposal team carried out a controlled explosion on a device attached to a drone that landed on Sunday evening in the Black Sea town of Tyulenovo in Bulgaria.
“We can certainly assume that it (the drone) is related to the war that Russia launched against Ukraine,” Defence Minister Todor Tagarev told reporters.
“This war is inevitably associated with increasing risks to our security,” he added.
Tagarev did not provide more detailed information on where the drone came from and how it reached Bulgaria.
The resort of Tyulenovo is 43 miles south of the Romanian border and across the Black Sea from Crimea, the Ukrainian peninsula forcibly annexed by Russia in 2014 and now a regular target of Ukrainian drone attacks.
Police cordoned off the area on Sunday evening and restricted public access to local restaurant terraces, Marian Zhechev, mayor of Shabla municipality of which Tyulenovo is a part, told Nova TV.
He said the drone had been found on rocks next to moored boats at Tyulenovo, describing it as an “aircraft with standard ammunition”.
It was unclear whether the drone had fallen from the air or been washed up by the sea.
A local news web site quoted witnesses saying the drone was between 3 and 3.5 metres long and had a container with explosives attached to it.
Last week fragments of a suspected drone were found in Romania, after a new Russian attack on Ukraine’s Danube ports across the border.
Russia invaded Ukraine last February. Ukraine launched a counteroffensive in June that so far has been marked by small victories but no major breakthroughs.
Despite being bolstered by NATO-standard weapons worth billions of dollars, Ukrainian military officials have said there are no quick solutions to puncture Russian defensive lines — only slow, grinding battles that have led to heavy losses.