The Ukrainian military fears Belarus "could invade tonight" after Russian leader Vladimir Putin met with Europe's last dictator Alexander Lukashenko.
This would mark a serious escalation 15 days into Putin's invasion of Ukraine.
Kyiv's Centre for Strategic Communications has said Belarusian troops may be drawn into invasion by 7pm local time, according to early data.
It comes as the Ukrainian government has accused Putin of staging "false flag" air attacks on Belarus from Ukraine to justify an intervention.
They say two Su-25 bombers crossed the border from Belarus on Friday afternoon, carrying out attacks on Ukrainian soil, before bombing Belarus itself.
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Ukrainian Defence Minister Olexiy Reznikov shared a Twitter thread detailing Putin's plans to run "false flag" air strikes from Ukrainian airspace.
Addressing the people of Belarus, he said Ukraine is not planning an offensive with Belarus and warned that Putin was "trying to bind you with blood".
An adviser to Ukraine's president, Volodymyr Zelensky, said that Belarusian strongman Lukashenko was in a meeting with Putin in Moscow, at the time of the attack.
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The two countries have been close allies since the dismantling of the Soviet Union in 1991. The countries also share a border.
Lukashenko is considered to be a puppet of Putin by many and has been regarded as Europe's last dictator.
Belarus has been a staging post for Russian forces since before the invasion on February 24.
Putin and Lukashenko agreed on Friday that Moscow would supply Belarus with up-to-date military equipment and on mutual support against Western sanctions, including on energy prices, the official Belarus Belta news agency said.
Mr Reznikov wrote: "I would like to address the Belarusian people. Based on the information in which we are fully confident and which we have confirmed, at this very moment, the command of the Russian occupation forces is preparing a series of bloody provocations.
"According to their criminal intent, the Russian aviation is preparing to launch a strike at several settlements on the territory of the Republic of Belarus located near the Ukrainian-Belarusian border. In particular, they are planning to hit the settlement of Kopany, Brest oblast.
"To mask this crime, Russia intends to carry out the strike from Ukrainian airspace. The purpose of this provocation is to force the current leadership of Belarus into war against Ukraine. Moscow is trying to bind you with blood.
"I guarantee that the Ukrainian army did not plan, is not planning and will not plan any aggressive actions against Belarus. During 15 days of the war, the Ukrainian armed forces liquidated over 12,000 Russian soldiers.
"Do not let your country be dragged into this war! Save the lives of your loved ones! Do not let the Kremlin destroy the future of Belarus!"
Putin recently said that there had been "certain positive shifts" in talks between his country and Ukraine recently, adding that they continued "practically on a daily basis".
Belarus has reportedly been intimately involved in the invasion of Ukraine, with the country north of Ukraine serving as a staging ground for Russian troops in the months preceding the invasion.
It has also hosted diplomatic talks between the two.
