DNIPROVS’KE, Ukraine — The smell of sawdust hangs in the air around a network of carefully dug trenches in a quiet, heavily forested area on the Ukraine-Belarus border.

Freshly cut planks reinforce the five-foot-high earthen walls in channels that crisscross the forest floor. Every few meters, the trunks at the top form a kind of shelter, the makeshift roof covered with branches and earth for camouflage.

Ukraine is preparing for battle in this quiet forest clearing less than a five-minute drive from the border with Russia’s close ally. Originally built in April, Ukrainian forces continue to upgrade and strengthen defenses like these trenches, amid reports of Russian troops and armor arriving in Belarus.

Belarus and Russia began joint military exercises on Monday, adding to fears that Moscow will use its ally to launch a new ground offensive, as it did with the invasion in February. The air force exercises will take place from Monday to February 1 using all of Belarus’ military airfields and joint army exercises involving a «mechanized brigade subdivision,» the Belarusian Defense Ministry said.

Standing by an empty trench, Lieutenant Mr. Anton knows that he and his men must be on high alert: the next attack could strike anywhere along the front lines that stretch thousands of miles.

“In all areas of our responsibility we are building these fortifications to be able to defend and mount counter-offensives clandestinely,” said the 32-year-old father of four and a former civil border guard. “The offensive potential of the Belarusian side could come from anywhere. That is why we are preparing for all scenarios”.

The officer uses only his first name because he is on active duty, a reflection of the heightened security along the border. Another is that the soldiers who built the trenches were temporarily evicted from the area before NBC News arrived for security reasons.

Passing through the city of Chernihiv, the nervous police demanded identification documents from NBC News. There seemed to be more military personnel than civilians on the streets.

Still, a Russian push is more likely to come from occupied territory in the east, Vadym Skibitsky, Ukraine’s deputy head of military intelligence, told NBC News.

“Now we are focused on the reserves and groupings of troops that Russia is placing in the temporarily occupied territories. We are talking about the spring-winter period, in three main directions: Donbas, Kharkiv and Zaporizhzhia,” Skibitsky said on Thursday.

Across the Dnipro River from Belarus, not far from the trenches, the Ukrainian forces take no chances.

A large bridge between the two countries sank into the icy water below, having been blown up by Kyiv forces to block Russia’s advance shortly after the February invasion.