Russia must allow Nato monitors to observe its military exercises near Baltic states next week to ensure they are not being used as a disguise or precursor for aggressive action against neighbours, the alliance's secretary general has said.
Jens Stoltenberg said Russia must accept Nato observers under guidelines agreed between the two powers for exercises of more than 30,000 troops.
He dismissed Moscow's claims that the Zapad 2017 exercise with Belarus would feature less than 30,000 troops, claiming Russia was "under-reporting" and "using loopholes" to avoid scrutiny, with around 100,000 estimated to be taking part.
Mr Stoltenberg also warned that exercises in the past have been used as a way to build up to invasions of other territories.
The exercise will run from September 14 to 20 not far from the borders of Nato allies Estonia, Latvia, Lithuania and Poland.
Britain has a battle group in Estonia, while Nato has stationed troops in the other Baltic states and Poland.
Mr Stoltenberg told BBC One's Andrew Marr Show: "Every nation, also Russia, has the right to exercise its forces.
"The problem is they are not doing that in a transparent way and we have seen before that Russia has used big military exercises as a disguise or a precursor for aggressive military actions against neighbours.
"That happened in Georgia in 2008 when they invaded Georgia and it happened in Crimea in 2014 when they illegally annexed Crimea.
"So we call (on) Russia to be fully transparent."
Defence Secretary Sir Michael Fallon said: "This is the biggest exercise I think for four years, over 100,000 Russian and Belorussian troops now on Nato's border.
"This is designed to provoke us, it's designed to test our defences, and that's why we have to be strong."
He added: "Russia is testing us and testing us now at every opportunity, we are seeing a more aggressive Russia, we have to deal with that."