Some 2,000 refugees began crossing into Macedonia on Thursday in freezing temperatures after the country conditionally reopened its border with Greece, after closing it temporarily.
"The border crossing for migrants [refugees] near (Macedonian border town) Gevgelija opened early this morning, but only those migrants [refugees] whose Greek registration papers show their final destination as Germany or Austria can enter," a senior police official in Skopje said.
Most of the refugees who fled Syria, Iraq and Afghanistan due to conflicts, were sleeping in heated tents set up by aid groups, including families with infants and children.
After Macedonia temporarily sealed the border this week, aid groups warned that their resources had been stretched to capacity.
"If this flow continues there is no possibility for accommodation," Antonis Rigas, head of the local Doctors Without Borders mission to the Greek side of the frontier, said earlier.
"It gets very cold at night. Early this morning the temperature was minus seven Celsius (19.4 Fahrenheit)," Rigas added.
About 600 refugees had spent the night on buses parked a few kilometres (miles) from the border.
On Wednesday, Macedonia declared that it sealed the border with Greece due to problems with Slovenian trains that had disrupted the flow of refugees heading further north.
However, the Slovenian rail company Slovenske Zeleznice insisted that they were running services as normal.
Greek police also stated that the frontier had actually shut a day earlier.
Some of leading children’s charities warned that young refugees were at risk from freezing due to Balkan weather.
Serbia and Croatia announced on Wednesday that they would do the same as Macedonia, allowing through only those who seek refugee in Austria and Germany.
Austria also said last week that it would follow Germany’s lead and begin turning back any arrivals seeking to claim asylum in Scandinavia, after Sweden and Denmark tightened their borders.
Countries along the Balkan route had earlier restricted entry, allowing only refugees from Syria, Afghanistan and Iraq.
In 2015, more than one million refugees crossed the Mediterranean Sea to head to Europe, almost half of them were Syrians, according to the UN refugee agency.
According to figures announced by the International Organisation for Migration this week, 31,000 refugees had arrived in Greece already this year.