Rows of white concrete barricades and coils of razor wire stretch across an open field for more than a kilometre.
Trenches with rudimentary living quarters are being dug under the cover of darkness as artillery rumbles not far away.
New defensive lines visited by Reuters near the northeast city of Kupiansk on December 28 show how Ukraine has stepped up construction of fortifications in recent months as it shifts its military operations against Russia to a more defensive footing.
The defences, which bear some similarities to those rolled out in the Russian-occupied south and east, aim to help Ukraine weather assaults while regenerating its forces as Moscow takes the battlefield initiative, military analysts said.
"As soon as the troops are moving, traversing fields, you can do without fortifications, but when the troops stop, you need to immediately dig into the ground," a Ukrainian army engineer with the call sign Lynx told Reuters near Kupiansk.
President Volodymyr Zelenskiy announced Ukraine was "significantly enhancing" fortifications on November 28 after a counteroffensive it had launched in June was unable to rapidly punch through Russian lines.
Kyiv says it is unswayed in its ambition to retake all remaining occupied territory, but for now, is focused on politically sensitive conscription reforms to replenish manpower and on addressing artillery shortages at the front.
Russia has been ramping up offensive pressure around eastern towns such as Kupiansk, Lyman and Avdiivka, and no longer needs to hold back its reserve troops for fear of a possible Ukrainian breakthrough, the military analysts said.
Zelenskiy said Ukraine's defensive constructions needed to be boosted and work on them accelerated around the three towns, in eastern parts of the Donetsk region, and the regions of Kharkiv, Sumy, Chernihiv, Kyiv, Rivne and Volyn.
Those regions stretch up from Ukraine's east, along the border with Russia and Belarus, to its western ally Poland.
Zelenskiy said the southern Kherson region, a swathe of which is still occupied, would also be reinforced.
Ukraine has had defensive lines in some areas of the eastern Donbas region since 2014 when Russia-backed militants seized territory.
It has been heavily dug in at places such as Avdiivka throughout the full-scale invasion.
Stronger fortifications would slow down Russian troops and suck fewer Ukrainian forces into defence, freeing them up from the front so they could, for instance, receive more training, said Jack Watling, senior research fellow for land warfare at the Royal United Services Institute.
"The Ukrainians are now shifting onto a defensive posture because their offensive has culminated," he said in a telephone interview, adding that Russia had retaken the initiative on the battlefield and was able to choose where to attack.
With Ukrainian artillery ammunition stocks declining, the rate of Russian casualties was falling, making it easier for Moscow to generate new units, which in time could allow them to open up new lines of attack, he said.
"On the Ukrainian side, they are trying to minimise their own casualties, but also regenerate offensive combat power," said Watling.
He said fortifications could also be used to defend Ukraine's flanks when it goes back on the offensive.
On Wednesday, Reuters reporters visited trenches being dug with an excavator and shovels at an undisclosed location in the Chernihiv region near the Russian border.
"When the civilians have done their job (building the positions), we will densely mine it," Serhiy Nayev, Ukraine's joint forces commander who oversees the northern military sector, told reporters at the site.
