Nine people were injured when Croatian police opened fire on a van carrying 29 mainly Afghan and Iraqi migrants that tried to crash through a roadblock after entering the EU from Bosnia, police say.

Two children were in a serious but stable condition and the others more lightly injured after the crash, which took place overnight near the south-eastern town of Zadar, police said.

Police had set up the roadblock to stop the van after it crossed into the country illegally, they said.

The driver, believed to be Bosnian, fled the scene and a search was underway for him.

"The driver did not stop after several warnings. He finally passed through a roadblock and drove towards the police," Zadar police chief Ranko Drazina said.

"The police reaction was justified. They were forced to open fire because their lives were in danger."

The main Balkan route that saw more than a million migrants enter the European Union in 2015 was shut in 2016 by an agreement between the EU and Turkey.

But thousands have entered this year through a new route opened by smugglers, running from Greece via Albania, Montenegro, Serbia and Bosnia to Croatia.

Croatia is the youngest member of the EU, which it joined in 2013. Bosnia has applied to join.

Bosnian and Croatian police have arrested dozens of smugglers in several operations in recent months.

Officials in Bosnia said this week about 5,000 migrants had entered the country this year.

Border police have stepped up controls along the 1,000 kilometre-long border.

Reuters