Meetings between the top US defence official Elbridge Colby, and five European countries have raised concerns that such forms could create divisions within NATO.
The group, known as RHergen Group, includes Sweden, Finland, Poland, Germany and the Netherlands.
Britain, France and the Baltic states are not part of it.
NATO experts warn that the creation of individual groups could create the impression that the US is favouring some allies over others.
According to them, it becomes especially sensitive, while Washington is demanding that Europe assume more responsibility for protecting the continent.
NATO, meanwhile, has downplayed the importance of meetings, saying it is normal for allies to hold meetings in smaller formats.
The lack of ballistic sites is particularly evident, as they are among the members who spend more on defence and are on the front lines facing Russia.
Lithuania, meanwhile, has invited Colby to visit the region to take a close look at air space violations and recent incidents that authorities link with hybrid threats from Russia and Belarus. /Telegraphy/