Techniques such as "misinformation, subversion, manipulation, assassination and of course, the use of mercenaries, which are easily undeclared" are the biggest danger, Carter said in an interview with the BBC, according to The Moscow Times.
"You can see how escalation (tension, n.r.) can be a possibility"Carter added.
Russia could thus trigger a Third World War, according to the British official.
US Secretary of State Mike Pompeo said that in order to deal with authoritarian regimes such as China and Russia, a joint effort by NATO countries is needed. London will host a NATO summit next month to mark the Alliance's 70th anniversary.
Meanwhile, French President Emmanuel Macron believes that NATO is "brain dead" and has questioned the basic commitments of the alliance, partly because of the US attitude under Donald Trump.
Carter said NATO is "An extremely successful alliance", which is changing its strategies to combat threats.
The British Parliament's Information and Security Committee has prepared a report on Russia's alleged interference in the 2016 UK referendum, but Prime Minister Boris Johnson has blocked its publication until after the elections, scheduled for December 12.
Carter said he could not comment on this, but it is understandable that people are concerned about how misinformation can affect the democratic process.