Thousands of refugees are stuck on Greece's border with Macedonia, overflowing from a packed camp into the surrounding fields, as they wait for Macedonian authorities to let them continue their trek through the Balkans.

Police say about 6,500 people are at or near the Idomeni border crossing, with another 500 moved to a hastily erected camp on a small concrete landing strip some 20 kilometers (13 miles) away.

Macedonian authorities let 300 Syrians and Iraqis in between 23:00 local time Sunday and 04:00 Monday, after which the crossing closed. Macedonia has said it will only allow in as many people as Serbia, the next country north on the Balkan migrant corridor, accepts.

This has led to a huge bottleneck in Greece, where authorities say more than 22,000 people are stuck.