The number of migrants and refugees entering Macedonia from Greece has risen significantly, with 10,000 making the crossing in just the past 24 hours, police say.
Since early September, the border has normally seen around 3000 to 5000 daily crossings by migrants, many of them fleeing conflict in the Middle East, heading to northern Europe hoping to start new lives.
Officials and charities have been expecting the number of migrants to fall as winter sets in and the sea crossing from Turkey to Greece becomes more dangerous - already nearly 3,000 have died or disappeared trying to cross the Mediterranean this year.
But Macedonia's interior ministry spokesman Ivo Kotevski said the latest police figures showed a "dramatic increase" in people crossing into the small Balkan country.
"This huge migratory influx is expected to continue in the coming days," he told AFP.
"We are monitoring the situation, following all information and ... taking measures to deal with this phenomenon."
About 630,000 people fleeing conflict in Syria and other countries have illegally entered the European Union this year, according to the bloc's border agency chief.
Many of them land in Greece and travel through the Balkans to EU member Hungary, bound for northern Europe, especially Germany and Sweden.