Ukrainian troops got a crash course in modern guerrilla warfare from the Central Intelligence Agency following Russia’s 2014 invasion of Crimea — training that US officials believe is helping Ukraine hold off the Kremlin now.

CIA paramilitary officers with the Special Activities Division began secretly training Ukrainian forces in sniping, anti-tank warfare and surveillance evasion shortly after the 2014 incursion, Yahoo News reported Wednesday.

Half a dozen former intelligence officials told the outlet that they see hallmarks of their training in the ongoing Ukrainian resistance.

“I think we’re seeing a big impact from snipers,” one former intelligence official told Yahoo. “Especially as Russian forces get bogged down from lack of supplies, I think the training really paid off.”

“We tried to really focus on operational planning, then really hard military skills like long-range marksmanship,” another former CIA official said. “Not just the capacity to do it, but to know how to do it on a battlefield, to really deplete the leadership on the other side.”

The Ukrainians appear to have taken the lessons to heart, claiming to have killed four high-ranking Russian generals on the battlefield in three weeks of fighting.

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The CIA also trained Ukrainians to evade Russian electronic surveillance, which was making them sitting ducks for artillery fire in the trenches of the Donbas region.

Ukrainian soldiers “were using mobile phones in a trench,” one former intelligence official told Yahoo. “People were getting blown to bits.”

CIA agents had to develop secure systems so the Ukrainians could communicate, then move before “[the Russians] start[ed] rocketing the crap out of you,” the former official said.

The paramilitaries were pulled out of Ukraine last month as tensions with Russia grew, another former official said, noting there were no agency paramilitaries on the ground when Russia invaded at the end of February.