At least nine migrants have drowned after their boat capsized while trying to cross the Drina River, which separates Serbia and Bosnia.
Authorities said at least 18 other migrants, including three children, arrived safely on the Bosnian side of the border.
But teams from both countries are searching for several people still unaccounted for. Parts of the river are up to 200 metres wide.
Local media also reported a search is under way for a smuggler who took the migrants across the river.
Vlada Rankic, the leader of the civil defence rescue team, told reporters that rescuers were also searching for a mother and a baby.
“Unfortunately, we don’t think there will be survivors,” he added.
Bosnia’s border police confirmed the incident took place early in the morning, but did not provide more details.
Serbian officials also confirmed that an incident had taken place near the border town of Ljubovija, with police adding that the majority of the migrants were people from Morocco.
Serbia and Bosnia are among the main transit countries on the route through the Western Balkans into the European Union.
In September 2023, according to the EU’s border police organisation Frontex, the route overtook the Central Mediterranean route through Italy in numbers of irregular border crossings, with many migrants crossing through Bulgaria.
The Serbian government says over a million people from Asia and Africa have crossed into the country since the refugee crisis of 2015. More than one million migrants entered Europe that year, according to the International Organization for Migration (IOM).
According to government data, the majority attempting to cross into Serbia in recent months came from Syria, Afghanistan, Turkey, Morocco and Pakistan.
But the number of migrants transiting through Serbia has decreased significantly over the years.
Serbian police recorded 10,389 illegal entries in the first half of 2024, which is nearly 70% less than the previous year.
Serbian officials have credited the drop to tighter cooperation with Austrian police and with Frontex.
Many migrants use smugglers to enter Serbia from Bulgaria and North Macedonia and then try to cross into EU members Hungary or Croatia.