European countries are quietly developing a response plan for a large-scale hybrid attack that could cause heavy civilian casualties, Politico reported, citing sources.

According to the publication, EU officials are now discussing how member states should respond if such an attack takes place. Diplomats interviewed by Politico said the scenario is increasingly being viewed as realistic.

In recent weeks, ambassadors from all 27 EU countries reportedly discussed what was described as a nightmare attack scenario during restricted-format meetings held without phones or aides.

The European Union has previously worked on coordinating responses to cyberattacks, election interference and sabotage. More recent discussions, however, have shifted toward more severe scenarios and their possible consequences.

One source said talks between EU diplomats and ministers covered several areas, including the possible deployment of emergency response teams and the allocation of financial assistance to a country affected by an attack.

At the same time, some European politicians are concerned that new security initiatives from Brussels could overlap with NATO’s core functions and make decision-making more complicated. Doubts have also been raised over the security of EU institutional communications and the risk of leaks.

For that reason, many countries prefer to discuss the most sensitive information within NATO frameworks.