Ongoing Nato war games on Russian President Vladimir Putin's front lawn are not empty exercises, but a taste of things to come if Putin continues to cause trouble in the region, Dutch Foreign Minister Bert Koenders has warned.

"There are Nato member states in eastern Europe that feel threatened by what is happening in eastern Ukraine. These drills are also a warning to President Putin," Koenders declared.

The warning comes after UK Defence Secretary Michael Fallon issued a similar alert to Putin. He said the military exercises are "not a game."

"This is a very timely demonstration of our commitment to collective defence," said Fallon. "It's a warning to Putin that his sabre rattling will not undermine Nato or Britain's resolve to stand shoulder to shoulder with our allies. This exercise ... demonstrates Nato's alertness to the danger and its determination not to be intimidated by that kind of sabre rattling."

Tit-for-tat threats have mounted in recent days as the rhetoric continues to crank up and relations between Russia and the West crumbles amid the Ukraine crisis.

Fallon's comments were triggered by Putin's announcement this week that Russia would expand its nuclear arsenal with 40 new intercontinental ballistic missiles. That announcement at a Moscow arms fair came after Russian officials denounced US plans to station heavy military equipment in central Europe and threatened to take action to stop the operation.

The region around Russia has become a nearly unprecedented centre of activity for Nato war games and parallel Russian exercises the last few months.

The current BALTOPS war games, which end on 20 June, include 14 Nato members plus three nations not part of the alliance -- Sweden, Finland and Georgia — which may soon join.