As three informed officials have confirmed to the Financial Times, the incident is considered a Russian interference operation.

GPS was not working in the entire Plovdiv airport area, one of the officials said.

After circling the airport for an hour, the pilot made the decision to land manually using paper maps, the source added.

"It was an indisputable interference," another official stressed.

The Bulgarian air traffic control service confirmed to the newspaper that such an incident had occurred.

"Since February 2022, serious cases of [GPS] jamming have been observed, and more recently, signal spoofing," the Bulgarian air traffic control service said in a statement.

"We are well aware that threats and intimidation are a regular part of Russia's hostile actions," an EC spokesman said."This will further strengthen our unwavering commitment to increasing our defense capabilities and supporting Ukraine."

Leyen flew from Warsaw to Plovdiv on Sunday to meet with Bulgarian Prime Minister Rosen Zhelezkov and visit a munitions factory.