Around 4,000-5,000 migrants were attempting on Thursday to board trains to the Croatian capital Zagreb from the small town of Tovarnik after crossing the border with Serbia, the UN refugee agency said.

"There are between 4,000 and 5,000 people here," Jan Kapic, a UNHCR spokesman, said from Tovarnik station in eastern Croatia. "Trains are coming but they can't take all these people."