The number of Russian military planes flying through European airspace has reached an “unusual” amount this week, according to NATO.

On Wednesday, eight Russian aircraft were detected over the North Sea. Those included four bombers and four tanker aircraft flying in formation, according to a NATO statement. Fighter jets from Norway, the United Kingdom, and Portugal were scrambled in response and tracked the planes.

“The bomber and tanker aircraft from Russia did not file flight plans or maintain radio contact with civilian air traffic,” the statement says. “This poses a potential risk to civil aviation as civilian air traffic cannot detect these aircraft or ensure there is no interference with civilian air traffic.”

Several other Russian planes were intercepted over the Black Sea and Baltic Sea over the past two days.

“These sizable Russian flights represent an unusual level of air activity over European airspace,” NATO said.

The unannounced aircraft could cause problems with civilian air traffic, since “Russian military often do not file flight plans, or use their on-board transponders” and can therefore not be detected by civilian air traffic control.

Tensions between Russia and its European neighbors have become increasingly strained this year following its role in destabilizing Ukraine. Moscow annexed Crimea this spring and has boosted the separatist rebels in eastern Ukraine, leading to sanctions and heated rhetoric with the West that have recalled the Cold War.

Last week, Russian President Vladimir Putin sharply criticized the United States in a speech to a group of Russia experts in Sochi. He accused the U.S. of trying to “remake the whole world” and undermining security in eastern Europe.

“We did not start this,” Putin said, according to The New York Times. “Statements that Russia is trying to reinstate some sort of empire, that it is encroaching on the sovereignty of its neighbors, are groundless.”

Putin has insisted that Moscow’s role in Ukraine is a response to a coup by Western-backed forces.

According to NATO, it has intercepted Russian planes over Europe more than 100 times this year. That is a three-fold increase over 2013.

[Photo Credit: Sergey Krivchikov – Russian AviaPhoto]