Thousands of migrants gathered at the border between Macedonia and Serbia on Sunday, after Macedonian officials allowed them the day before to cross into their territory following days of high tension and clashes.

The move is likely to add to the pressure on Serbia and Hungary, two main transit countries for Middle Eastern and African migrants attempting to reach Northern Europe.

During the night, about 40 private buses ferried the migrants from the train station in Gevgelija, near the country’s southern border, to Tabanovce, at Macedonia’s frontier with Serbia. In normal driving conditions it takes around two hours to cover the distance.

Authorities had also set up at least two trains to cross the country. In recent days, at the Gevgelija station, taxi drivers were available to take people north for about €100 ($114), according to migrants interviewed there.

In letting the migrants enter relatively freely, Macedonia reversed an earlier decision to effectively seal the border with Greece, leaving thousands stuck on the Greek side in difficult conditions.