While the official would not speak to Russian President Vladimir Putin's intent when it comes to moving troops into Belarus, the official described Putin as "an opportunist."
"We've seen warning signs that the dynamics inside Belarus are enabling Russia to further prey on Lukashenko's self-inflicted vulnerability," the official said.
US officials have said a Russian invasion of Ukraine could happen at any point in the next month or two.
"Russian military plans to begin activities several weeks before a military invasion are something we've been watching closely and our assessment has been that could happen anytime between mid-January and mid-February," a second senior State Department official explained.
The United States is looking closely at whether Lukashenko still has the levers of control in his country -- or whether the decision-making has been largely passed over to Russia.
Belarus has become an "increasingly destabilizing actor in the region" the first State Department official said, pointing to several recent actions such as manufacturing a migrant crisis on the Poland-Belarus border, arresting activists, and holding more than 900 political prisoners.
'United against Putin'
Ukraine's assessment comes as the country's former President, Petro Poroshenko, told CNN's Christiane Amanpour on Tuesday that "the whole world should be united against Putin," and that Ukraine should be allowed to join NATO.
The billionaire led Ukraine from 2014 until 2019, taking power shortly after Russia invaded and subsequently annexed Crimea. He was defeated in the 2019 presidential election by incumbent Volodymyr Zelensky.
"There must be international solidarity and unity" against Putin, Poroshenko said, adding that Western allies should not trust the Russian leader.
The 56-year-old also called for an increase in international sanctions against Russia. "We need to make Russia weaker, and to make Russia weaker, we can do that through the sanctions. We should make Ukraine stronger. And day by day, we should receive new effective defensive lethal weapons," he said.
Poroshenko also said "nobody knows, including Putin" whether a Russian invasion will happen and a lot will depend on the introduction of sanctions. Calling a potential invasion a "crazy decision," he said the international community should "increase significantly the price Putin should pay" if Russian forces cross the Ukrainian border.
When asked whether endemic corruption is a reason Ukraine has not been accepted as a member of NATO, Poroshenko pointed blame at his successor, Zelensky, for a "backslide" of corruption reforms.
Poroshenko returned to the capital, Kyiv, on Monday to face treason charges linked to the financing of Russian-backed separatist fighters through illegal coal sales in 2014 and 2015.
When pressed on the charges he faces, Poroshenko said the accusations were "politically motivated" and that prosecutors had "zero evidence."
According to Reuters, critics say his return to Ukraine is serving as a badly timed distraction amid the political crisis with Russia.