"Only an insane person" would believe Russia could attack Nato after it seized the Crimea from Ukraine, Vladimir Putin has said.
The Russian president said he could imagine Russia "only in a dream" attacking a country that is in the transatlantic alliance and thereby possibly triggering World War Three.
Every member of Nato, including the UK and the USA, is obliged to intervene to defend any other member nation if it is attacked.
In the last year, countries including the UK have deployed forces to bolster the defence of Nato members that share a border with Russia, as tensions between the country and the West have escalated.
In an interview with Italian newspaper Il Corriere della Sera, Putin said: "As for some countries’ concerns about Russia's possible aggressive actions, I think that only an insane person and only in a dream can imagine that Russia would suddenly attack NATO.
"I think some countries are simply taking advantage of people’s fears with regard to Russia. They just want to play the role of front-line countries that should receive some supplementary military, economic, financial or some other aid.
"Therefore, it is pointless to support this idea; it is absolutely groundless. But some may be interested in fostering such fears. I can only make a conjecture."
Putin was speaking as world leaders gather in Germany for a summit of the G7, where the strategy towards Russia is likely to be a big topic for discussion.
Russia's actions in Ukraine have seen economic and diplomatic sanctions placed on it but this has not appeared to alter its military posture.
US Defence Secretary Ash Carter said on Friday that this was "a sign of how heedless the Russian government seems to be about the long-term welfare of its own people that it has not yet resulted in a change, in a reversal at least of course, which is what we want out of Russia".
In his interview, Putin was asked whether Russia would ever intervene in eastern Ukraine to annex or occupy it, as it did in the Crimea. Pro-Russian separatists fighting there are widely believed to be receiving material support from Russia.
Putin said: "You know, the Crimean scenario does not reflect Russia’s position; it reflects the position of the people who live in Crimea.
"All our actions, including those with the use of force, were aimed not at tearing away this territory from Ukraine but at giving the people living there an opportunity to express their opinion on how they want to live their lives."