Hungarian riot police charge migrants at the border crossing with Serbia in Roszke, Hungary, September 16, 2015. [Photo/Agencies]

"It is getting very ugly there," said Ahmad, 58, a shopkeeper from Baghdad who went to the official border crossing at Sid in Serbia but realised he may have a better chance of entering the EU via Serbia's border with Croatia.

"As soon as we heard about a route to Croatia we did not wait long. I want to go to Sweden to meet the rest of my family. I hope we will be treated better in Croatia," he told Reuters.

Serbia's prime minister accused Hungary of "brutal" and "non-European" behaviour and urged the EU to respond.

"I call on the European Union to react, for its members to behave in line with European values," Aleksandar Vucic told Serbian state television during a visit to the United States.

"If the EU does not react, we will find a way to protect our borders and European values as well," he said.

Serbia said it would send extra police to the border and try to distance migrants from the fence, as three Hungarian military Humvees mounted with guns stepped up security on the other side.