▶ Watch Video: NATO and Russia reach an impasse in talks on Ukraine

The United States has information indicating Russia prepositioned a group of operatives to conduct a false-flag operation in eastern Ukraine – groundwork for a possible “pretext for invasion,” according to a U.S. official.

The official told CBS News on Friday the Russian operatives were “trained in urban warfare and in using explosives to carry out acts of sabotage against Russia’s own proxy-forces,” and could take action “several weeks before a military invasion, which could begin between mid-January and mid-February.”  

The National Security Council declined to specify where the Russian teams were thought to be located.  

Online, Russian actors have also been fabricating “Ukrainian provocations in state and social media to justify a Russian intervention and sow divisions in Ukraine,” the official said, noting there was a 200% increase last month in Russian-language narratives blaming the West and Ukrainian authorities for escalating tensions.  

National Security Advisor Jake Sullivan said on Thursday the moves were reminiscent of Russia’s actions eight years ago, ahead of its invasion and occupation of Crimea.  

“We saw this playbook in 2014. They are preparing this playbook again,” Sullivan said.  

He said the U.S. is ready to respond to Russia if it abandons diplomatic talks in favor of military aggression, but said the U.S. intelligence community had not determined that Moscow had “definitively decided” to mount a Ukraine invasion.

“We’re ready to make progress at the negotiating table – serious, tangible progress on important issues of concern to us, to Europe, and to Russia in an environment of de-escalation,” Sullivan said. “And we’re ready to take the necessary and proper steps to defend our allies, support our partners, and respond robustly to any naked aggression that might occur.” 

His comments followed several days of intensive but largely fruitless diplomacy among the U.S., NATO, European nations and Russia, which has amassed 100,000 troops at its border with Ukraine and recently conducted live-fire drills with troops and heavy artillery.  

Moscow has for weeks been demanding security guarantees from the West that include a commitment that Ukraine will never be allowed to join NATO. 

NATO and the U.S. have repeatedly dismissed that demand as a “non-starter,” while signaling a willingness to engage with Russia on issues related to arms control, missile placement and transparency into military activities.  

On Friday, Ukrainian officials said multiple government websites had suffered a “massive cyberattack” that downed them temporarily. President Biden was briefed on the attack but the U.S. has not yet determined who was behind it, said a National Security Council spokesperson.

Just hours earlier, Ukraine’s ambassador in Washington told CBS News her country believed a cyberattack would precede any major military action by Russian President Vladimir Putin’s forces.

NATO Secretary General Jens Stoltenberg condemned the cyberattacks and said the alliance was actively “supporting the Ukrainian authorities on the ground.” He also said NATO and Ukraine would sign an enhanced” agreement on cyber cooperation in the coming days.