Resistance movement growing in captured areas in southern Ukraine - CNN.

A trio of assassination attempts targeting pro-Russian officials over the past two weeks suggests a burgeoning resistance movement against pro-Russian authorities occupying parts of southern Ukraine, according to U.S. officials.

That's according to CNN, Ukrinform reports.

The resistance could grow into a wider counterinsurgency that would pose a significant challenge to Russia's ability to control newly captured territory across Ukraine. The Kremlin "faces rising partisan activity in southern Ukraine," Avril Haines, the director of national intelligence, said during a conference in Washington, DC, on Wednesday.

The U.S. believes that Russia does not have enough forces in Kherson to effectively occupy and control the region, one U.S. official said, especially after pulling forces from the area for the fight to the east in Donbas.

In addition, Ukraine has also conducted limited counterattacks near Kherson, further straining Russian forces.

The first attack in Kherson occurred on June 16, when an explosion shattered the windows of a white Audi Q7 SUV. The vehicle was left seriously damaged, but the target of the attack survived.

Less than a week later, a second pro-Russian official in Kherson was targeted. This time, the attack succeeded. On June 24, Dmitry Savluchenko, the pro-Russian official in charge of the Department of Youth and Sports for the Kherson region, was killed.

On Tuesday, the car of a third pro-Russian official was set on fire in Kherson, though the official was not injured.

In the long term, the U.S. assesses that Russia will eventually face a counterinsurgency from the local Ukrainian population.

"I think Russia is going to have significant challenges in trying to establish any sort of stable administration for these regions, because likely collaborators -- more prominent ones -- are going to be assassinated and others will be living in fear," said Michael Kofman, director for Russia studies at the Center for Naval Analyses, a Washington-based think tank.