BELGRADE, Serbia (AP) — Serbia on Wednesday introduced a lockdown for migrants in their refugee center outside Belgrade after an alleged attack against a woman walking with her children.

The measure comes amid widespread claims that the migrants are being forcefully pushed back and beaten when they try to enter Hungary and Croatia, both European Union member states.

Serbian officials said the migrants staying in the center on the outskirts of the capital will need special permissions to leave and need to be back by 10 p.m. each night.

The measure has been introduced after a woman claimed she was attacked by migrants near the camp while walking with her three children. She said they wanted to take away the stroller with her baby inside.

Officials said police are investigating her claim.

“We are establishing tougher control, meaning that only with certain permissions they would be allowed to leave the camp, to return at a certain time and that they would be issued appropriate identity papers,” said Aleksandar Vulin, a government minister in charge of the migrants.

About 550 migrants, mostly from Pakistan and Afghanistan, are staying in the camp located in a former army barracks. It isn’t clear whether the measure includes several other shelters in Serbia.

Around 7,000 migrants have been stranded in Serbia following Balkan border closures last year and the European Union’s attempts to control immigration. Many are sleeping rough in makeshift shelters or parks.

The charity Save the Children said that hundreds of children and other migrants are regularly beaten by Hungarian and Croatian border guards when caught as they try to enter the EU.

The charity believes an average of 30 “clandestine push-backs” are happening each day, “with many refugees being denied their right under asylum law to an individual assessment of their need for international protection.”

Tahir Khan, a migrant from Pakistan stranded near the border with Hungary, said he and several others have tried to cross seven times in three months.

“When we cross the border sometimes they (Hungarian police) catch us, and then beat us and send us again to Serbia,” he told The Associated Press in an abandoned brick factory amid freezing temperatures. “And when we go there, we are walking three and four hours at a time.”