Croatia says it will allow migrants to travel on to northern Europe - opening up a new route a day after Hungary sealed its border with Serbia.
More than 150 migrants have crossed into Croatia from Serbia. Those stranded on Serbia's border with Hungary are making a similar trip.
Croatia says it is ready to receive them or "direct" them to where they want to go.
Many migrants - mainly Syrian - are hoping to reach Germany.
New border restrictions and a row over allocating migrants have shown bitter divisions in Europe over the crisis.