Belongings of migrants are pictured in an underground station at the Keleti trainstation in Budapest, Hungary, September 4, 2015. [Photo/Agencies]

The migrants left shoes, clothes and mattresses scattered behind them. Helicopters circled overhead.

Austria's Red Cross said it expected between 800 and 1,500 people to arrive in its refugee reception centre of Nickelsdorf at the Hungarian border overnight.

"Because of today's emergency situation on the Hungarian border, Austria and Germany agree in this case to a continuation of the refugees' journey into their countries," Austrian Chancellor Werner Faymann said on his Facebook page.