Authorities said Thursday they had retrieved the bodies of three migrants who had died while trying to cross the river on Serbia’s western border with Bosnia. Some reports said four bodies had been found. A dozen others remained missing, Boris Trninic, the head of the regional civil protection administration in Bosnia-Herzegovina, told the Srna news agency.

"Since all four bodies were found closer to Serbia, Serbian police have taken custody of the bodies," Trninic said.

He added that around 30 people were believed to have been on board the boat.

According to reports, between 15 and 18 of the passengers, including several children, managed to reach the riverbank.

In a statement, Serbia’s interior minister, Ivica Dacic, said police in Serbia had learned of the incident during the night after they were contacted by the Bosnian authorities, as well as by a local resident.

The search for the remainder of the missing migrants continued on Thursday.

Also read: Migrants 'dying without a trace' on the Balkan route

Drop in migrants transiting Serbia

Serbia is one of the main countries on the so-called 'Balkan route' through which migrants pass in an effort to reach the EU. It borders four member states – Croatia, Romania, Bulgaria and Hungary.

According to the Serbian government, most of those who have tried to cross in recent months come from Syria, Afghanistan, Turkey, Morocco and Pakistan.

Over the past decade at least 40 migrants have died trying to cross the Drina to Bosnia, a Radio Free Europe/Radio Liberty report said Thursday.

More than a million people have transited through Serbia since 2015, according to the Serbian government, however, the number has dropped significantly in the past few years.

Serbian officials say this is due to cooperation with Austrian police and with Frontex, the EU's border agency.

With AFP, Reuters