BRUSSELS - Russia's drone incursions and airspace violation of European NATO countries has increased since September, when more than 20 Russian drones entered Polish airspace and three Russian military jets violated Estonia's airspace for 12 minutes.

Since then, many drone flights, the origin of which are mostly unknown, have disrupted airspace operations in Europe.

European Commission President Ursula von der Leyen has called the incursions "hybrid warfare", and while she has not said Russia was responsible for all the incidents, she said it was clear Russia's aim was to "sow division" in Europe.

Below are some of the biggest airspace violations and drone sightings reported so far this year:

BELGIUM

The Brussels and Liege airports were closed on Tuesday evening after drones were spotted, diverting many incoming planes and preventing others from taking off. The Brussels airport reopened Wednesday morning though some flights were cancelled and others delayed.

Belgian Defence Minister Theo Francken said on Sunday that police was investigating drone sightings over the Kleine Brogel air base in north-east Belgium.

The country opened an investigation last week after two drone sightings over a military base in the country's south-east, and another inquiry last month after several drones were spotted flying over a military base at Elsenborn, on the German border.

CZECH REPUBLIC

The Czech army said on September 10 it was detecting an increasing number of unidentified drones flying over its military facilities, according to the Institute for the Study of War research group.

DENMARK

Drones disrupted air traffic at six Danish airports in September, including in Copenhagen, the Nordic region's busiest airport, in what Prime Minister Mette Frederiksen called a hybrid attack on her nation.

ESTONIA