At least 250 heavily armed US troops on Monday began a military drill along with Bulgarian units which will last more than 40 days, officials said.
The exercise will continue until May 16 at Novo Selo Training Area, one of the four joint US-Bulgarian military bases in the Balkan country, the Bulgarian Defence Ministry said in a statement, Xinhua reported.
Tanks M1 Abrams and infantry fighting vehicles will be used during the drill.
By conducting such pre-planned joint exercises, the Bulgarian Army aims to maintain good combat readiness and achieve greater interoperability with NATO allies, the Defence Ministry said.
In 2015, military units of the Bulgarian army participated in 82 joint trainings and drills with the US Army, the ministry said.
Bulgaria, a NATO member since 2004, signed a defence cooperation agreement with the United States in 2006.
