In a forest outside Kyiv, dozens of masked volunteers practice guerrilla tactics and survival skills.
They are getting ready to support Ukrainian troops fighting pro-Russian rebels in the east of the country.
“We are preparing for war,” explained Georgiy, the group’s instructor.
“As I said in the very beginning, we are preparing them so that during occupation or invasion of Russian troops they will be able to survive in military conditions and destroy the enemy,” Georgiy added.
It is not clear whether the group of men will be called into action.
Ukrainian President Petro Poroshenko said Russia had taken most of its forces out of his country.
But Russia insists they were never there in the first place.
Kyiv and Moscow both agree a fragile ceasefire in eastern Ukraine is largely holding but tensions remain.
As trucks from a second Russian humanitarian aid convoy wait to cross the border into eastern Ukraine, Moscow said it is committed to help enforce a peace plan for Ukraine, despite EU sanctions.
Some European officials believe Russia may retaliate by disrupting natural gas supplies to the EU.