Work has begun in Hungary on a four-metre high fence to close the open border with Serbia.
Belgrade is strongly opposed to the plan which has also been criticised by the Council of Europe and the United Nations.
Stretching for 175 kilometres, the fence is designed to stem the influx of illegal immigrants coming up through the Balkans in the south and heading for the EU.
Figures suggest their numbers have almost doubled so far year-on-year.
More than 70 000 have been recorded in Hungary since the start of the year, the majority from the Middle East and North Africa.
That is compared to 43 000 for the whole of 2014.