Hungarian workers install razor-wire to a fence on the border with Serbia, which is intended to prevent illegal crossings into Hungary, near Asotthalom, southern Hungary, on Aug. 30, 2015. Bulgarian News Agency/Xinhua
Bulgarian and Hungarian police officers are performing joint patrols along Hungary's borders with Serbia as Budapest is seeking to stem the rising influx of migrants.
This follows arrests of Bulgarian nationals on suspicions of active involvement in a migrant smuggling ring.
On Sunday, Austrian police discovered the dead bodies of 71 migrants in a truck near the border with Hungary. Four Bulgarians and an Afghan man have been detained so far in relation to the incident.
The Central European nation is taking measures to deal with the tens of thousands of migrants who try to cross into the country every day. Some 140 000 have been detained while trying to enter since the year began, but tens of thousands make their way across the border every week.
Hungary last week completed the border fence two days ahead of schedule amid concerns over the swelling numbers of asylum seekers from Syria and elsewhere in the Middle East and North Africa.
Thousands of border police officers have been dispatched to guard the border, and the number is set to rise to 3000 in September.