Northern Ukraine — The SlabuUkrainian military has been conducting drills near the country's northern border with Belarus, aimed at showing preparedness to defend against any repeat of Russia's land invasion across that border. These drills on the northern front follow warnings that the Kremlin is planning a renewed offensive this spring.

CBS News visited the maze of trenches that Ukraine's forces built close to the border after Russian troops crossed over from Belarus as part of their failed push to capture Kyiv last year.

One of the Ukrainian commanders there said his troops were "fully ready," adding with a wry smile that they had "prepared a lot of presents" for any invading forces that might try to cross the border again.  

Commander Kent, a callsign, wouldn't give details about those "presents," but he made it clear that, unlike 12 months ago, Ukraine has a "a fortified border" capable of stopping heavy equipment and even light armor from "trespassing."

The trenches look like they could be right out of World War I, but the soldiers bracing for battle inside them have modern technology at their fingertips.

CBS News watched as a soldier used a reconnaissance drone to scour the skies and the border for any signs of trouble.

Several Ukrainian units are now spread across the 650-mile frontier with Belarus, but they have yet to detect any new mass mobilization of Russian forces in the neighboring country. U.S. officials are also watching what they say is a regular flow of Russian troops and hardware in and out of Belarus, but no signs of a new buildup there yet.

The Ukrainian forces are determined not to be taken by surprise. 

We asked the soldier flying the drone, who goes by the callsign Kraken, what he was spending six hours every day scanning for.

"Clusters of Russian military equipment," he said. "They need to gather somewhere… If we see any columns of equipment, we forward this information."

In Ukraine's east, however, the fighting is only intensifying. The industrial town of Bakhmut, right on the front line, has been the focus of much of the worst fighting this year. Once home to about 70,000 people, it has been virtually razed to the ground, and now nearly all of it is under Russia's control.

That has made protecting the north an even higher priority for Ukraine.

In:
• Belarus
• War
• Ukraine
• Russia
• Vladimir Putin
• Alexander Lukashenko

Imtiaz Tyab

Imtiaz Tyab is a CBS News correspondent based in London.

Twitter