Bulgarian officials Friday reported police had discovered eighteen dead migrants, including a child, in an abandoned truck in Lokorsko, 12 miles (19 km) northeast of the capital, Sofia.

Authorities made four arrests and transported 34 people to hospital, including five children, some in critical, but stable, condition. It is believed to be the country's deadliest incident involving migrants to date, with those alive "freezing, wet, [and] have not eaten for several days."

The truck was carrying timber with migrants in concealed compartments, according to the Ministry of Interior.

Health minister Asen Medzhidiev told reporters: "There has been a lack of oxygen to those who were locked in this truck"; according to head of the National Investigative Service Borislav Sarafov, some had suffocated.

Police believe the migrants were from Afghanistan and were being transported to Serbia through a route used by Middle Eastern migrants to reach the European Union. Sarafov said the migrants had illegally entered Bulgaria through Turkey and, after two days in the woods, entered the truck near Yambol.

One of the arrestees had already been sentenced for human trafficking; official Atanas Ilkov told Reuters charges were pending further evidence.

In December, Austria and The Netherlands blocked Bulgaria's accession to the passport-free Schengen Area over a rise in asylum-seekers entering through the Western Balkans and concerns the government was not addressing crime, respectively.

Both Bulgaria and its northern neighbour Romania, whose entry was also opposed by Austria, lie on the Western Balkan route for immigration, which Bucharest disputes.

The EU's external border control agency Frontex reported 61,375 migrants entered along that route in 2021, largely through Bulgaria or Greece, to the EU via common destinations Croatia, Hungary, Romania or Serbia.

The Ministry of Interior reported over 12,740 migrants were detained in Bulgaria in the first nine months of 2022, and attempted border crossings from January through August doubled to 80,000 from 2021 figures.