The European Union on Thursday demanded Hungary respond to fears that a decision by Budapest to relax visa rules for Russian and Belarusian nationals heightens the risk of spying in the bloc.

Hungarian Prime Minister Viktor Orban -- the only EU leader to have maintained close ties with the Kremlin since Russia launched its full-scale invasion of Ukraine in 2022 -- signed a decree earlier this month to extend its fast-track visa scheme to eight countries, including Russia and Belarus.

The immigration programme allows citizens from these countries to apply for so-called "national cards" to be able to work in Hungary. Previously, these cards were only available to Ukrainian and Serbian citizens.

The European Parliament's biggest group earlier this week complained the relaxed rules could create "grave loopholes for espionage activities" as Russia tries to target the EU amid the war in Ukraine.

The EU's home affairs commissioner Ylva Johansson said she agreed with the concerns and had sent a letter to Budapest requesting they respond to a list of questions by August 19.

"Russia is a security threat. We need more, not less vigilance. Giving potential Russian spies and saboteurs easy EU access would undermine the security of us all," Johansson wrote on X.

"Today in a letter I ask the Hungarian government to explain. If their easy access scheme is a risk, we will act."

Orban already enraged fellow EU leaders by jetting to Moscow on July 5 to meet Russian President Vladimir Putin, just days after his country took on the EU's six-month rotating presidency.