Up to 800 flights will have to be rerouted, and around 40 percent will have to fly 110 kms longer, according to estimates by the European Organization for the Safety of Air Navigation (Eurocontrol).

A passenger checks information on electric screens at Frankfurt airport in Frankfurt, Germany, Jan. 15, 2019. (Photo/Lu Yang, Xinhua)

Frankfurt - A massive NATO exercise that kicked off on Monday is set to cause major disruptions to passenger flights in Germany, raising concerns from air travellers and also protesters.

Jens Bischof, chief of Lufthansa Group's low-cost airline Eurowings, said he was concerned about the fallout of the exercise, which had led to the closure and heavy restriction of a large swath of German airspace.

The military drill, known as "Air Defender 23", is the biggest ever NATO air force deployment exercise involving 10,000 military personnel and 250 combat aircraft from 25 nations including the United States. It will last until June 23.

A plane of Lufthansa is seen at the Berlin Tegel Airport in Berlin, capital of Germany, May 26, 2020. (Photo/Shan Yuqi, Xinhua)

The two-week-drill will be carried out mainly in German airspace. The consequent impact includes possible delays of flights, prolonged flight time and cancellations at a busy time of year.

Up to 800 flights will have to be rerouted, and around 40 percent will have to fly 110 kms longer, according to estimates by the European Organization for the Safety of Air Navigation (Eurocontrol).

Hamburg Airport has told local media that it is impossible to know exactly which individual flights will be affected, due to the unprecedented nature of the ongoing exercises. Stuttgart Airport also put out a similar message on social media. Both suggested passengers to monitor the status of their flights closely.

The screenshot shows Stuttgart Airport's post on Twitter, saying: "The military maneuver 'Air Defender' begins today and will last until June 23, affecting civil aviation. Please check your flight status regularly and contact the airline if you have any questions."

Some 300 protestors rallied in front of an air base in the state of Lower Saxony on Saturday over the damage done to the climate by the aircrafts and over the NATO show of military might at a time when the conflict in Ukraine is raging on.

"If they want to use maneuvers to demonstrate, to intimidate others, this is always the wrong way," protester Thomas Wefing told Xinhua.

NATO has claimed that the drill is not targeted at anyone. However, Wefing believes it serves as a provocation to Russia and will lead to the escalation of the situation in Ukraine.

People take part in a protest against Sweden hosting a large international military exercise and the country striving to become a member of the North Atlantic Treaty Organization (NATO) in Stockholm, Sweden, on April 22, 2023. (Photo/Patrick Ekstrand, Xinhua)

Egon Roth, another protestor, shared Wefing's concern over the drill that it could lead to escalation, adding that conflicts can only be resolved through diplomacy rather than the use of weapons.

Jochen Scholz, a security expert and a former military officer, feared that the military exercise will be exploited to give Europeans the wrong idea that Russia poses a threat to the security of Europe.

"It is not Russia that is a threat to Europe, but the United States, because it sees domination of the Eurasian continent as a prerequisite for its global dominance," he said.