Ukrainian soldiers have conducted training near the border with Belarus to check the readiness of its units, according to the country's army.
"The danger exists because the Republic of Belarus is the ally of the Russian Federation in the aggression against Ukraine," said Serhiy Naev, a Commander of the Joint Forces of Ukraine.
It comes after Belarus president Alexander Lukashenko announced on Monday that his country's troops would group together with Russian soldiers. He did not offer any details of where the grouping might be deployed.
But, on Tuesday, Minsk clarified the common force would be "purely defensive".
Lukashenko's statement follows his repeated claims that Ukraine is plotting an attack on Belarus.
In more than seven months since Russia's invasion of Ukraine, there has been no indication that Kyiv's forces are planning an attack on Belarus
Meanwhile, officials in Warsaw have advised Polish citizens in Belarus to leave as tensions between the two sides intensify.
“We recommend that Polish citizens staying on the territory of the Republic of Belarus leave its territory with available commercial and private means,” the government said in guidance for travellers published on its website.