The Ukrainian government has allocated $509 million this year to fortify both its border with Russia and the front line with Russian forces across its east and south; Prime Minister Denys Shmhyal promised $142 million more on Tuesday.

In Chernihiv region to the north of the capital Kyiv, dozens of Ukrainian workers installed metal reinforcements and attached poles and plastic sheeting along a trench wall late last month while a digger cleared earth nearby.

"A bit further ahead is another line made up of 'dragon's teeth', pyramidal obstacles to stop military hardware," contractor Yuri Ilyashek said, describing the three-layered fortifications.

"And further still is the so-called anti-tank trench, very deep and very wide, hundreds of metres long."

Russian troops poured across the nearby border in February 2022, bearing down on Kyiv before being beaten back. The area has since been regularly shelled.

With Russian forces pushing hard in the east and vital U.S. military aid held up by months of Republican resistance in Congress, the urgency has mounted.

President Volodymyr Zelenskiy has warned that Russia may plan to mount a new offensive this spring or summer.

Ukraine has put down 100,000 "dragon's teeth" as part of defences stretching 2,000 km which allow for future advances, Oleksiy Kuleba, a deputy head of Zelenskiy's office who is overseeing the fortifications, told Reuters.

"The situation is dynamic. There will always be the need to reinforce, build additional fortifications or change them. Work will continue and go on even after our victory," he said.

In the southeastern Zaporizhzhia region, one of the hottest front line sectors, work goes on around the clock.

"Drones sent by the aggressor keep flying over us," said Maksym, an engineer and building inspector, as a crane lifts reinforced concrete into place and workers strip bark off logs.

"But we keep building. There is no other way."