Bulgaria will stage a series of military drills to support border police near the Macedonian frontier, Bulgaria's defence ministry said on Tuesday, amid an increased flow of migrants through the Balkan country’s neighbours. "The situation in the neighbouring states of Macedonia, Greece and Serbia is quite complicated and we have to react preventively," defence ministry's chief of staff Valeria Kardashevska told Reuters. More than 5,000 people crossed into Serbia from Macedonia on Sunday, resuming a journey to western Europe after Macedonia gave up its attempts to stem the flow of mainly Syrian refugees by force.