They believe the construction aims to stop future attacks by Ukrainian forces.
The Institute for The Study of War highlighting the barrier in the Donetsk province of eastern Ukraine.
“Russian forces appear to have constructed a 30-kilometer-long barrier dubbed the ‘tsar train’ in occupied Donetsk Oblast, possibly to serve as a defensive line against future Ukrainian assaults,” it said.
The barrier is reportedly constructed from over 2,100 freight cars.
The Washington-based military think tank also said Russian forces had made “confirmed advances” near Avdiivka, near Donetsk, and in western Zaporizhia province amid “continued positional engagements along the entire frontline”.
It also highlighted a report by CNN that Russia had recruited as many as 15,000 Nepalis to fight in Ukraine but that many of them had complained about poor conditions and lack of adequate training before their deployment to the “most active frontlines” in Ukraine.
Digital minister Mykhailo Fedorov said Kyiv will produce thousands of long-range drones capable of deep strikes into Russia in 2024.
Ten companies in Ukraine were already making drones that can reach Moscow and St Petersburg, he added, as part of a war effort to produce a million of the aircraft this year.
Mr Fedorov spoke about the wartime drone industry he has championed in a interview in Kyiv in which he revealed new details about the sector, after a spate of Ukrainian drone attacks on Russian oil facilities in recent weeks.
“The category of long-range kamikaze drones is growing, with a range of 300, 500, 700, and 1,000 kilometres. Two years ago, this category did not exist ... at all,” he said, as Vladimir Putin’s war in Ukraine is set to enter its third year.
Ukraine has trained 20,000 drone operators since it launched a programme to provide grants for military training in private schools at the beginning of 2023, he said. There were 20 such schools, he added.
“We pay for every military person who comes to these schools. Now we have a plan to turn this into a larger state programme and separately to modernize, update several training centres and make them work at a high level,” he explained.
Mr Fedorov, 33, has been at the heart of Ukraine’s effort to nurture private military start-ups to innovate and build up the drone industry as the war goes into its third year and Ukraine seeks new ways to fight back against well dug-in Russian forces.
The recent series of strikes on oil facilities, he said, reflected the government’s progress in rapidly deregulating the drone market and increasing funding for it, with the state acting as a venture investor.
Some $2.5 million (£2 million) in grants were allocated to military tech startups via the BRAVE1 initiative set up by the government last year, an amount set to be increased roughly tenfold in 2024, he said.
“We will fight to increase the financing even more,” he added.
In contrast with Russia where drone production is dominated by the state, the vast majority of manufacturers in Ukraine are private.
Mr Fedorov said only one of the 10 companies whose drones could fly as far as the regions around Moscow or St Petersburg was a state company.
Since the first year of the full-scale war, Russia has used thousands of Iranian “Shahed” drones that fly towards their target and detonate on impact for long-range strikes.
Ukraine’s production levels and deliveries increased more than 120 times in 2023, according to Mr Fedorov, part of a broader wartime push to develop and produce drones to narrow the gap with Russia’s strike capabilities.
He agreed with an assessment by Ukraine’s military spy chief Kyrylo Budanov that Kyiv had achieved a “certain kind of parity” with Moscow in the production of long-range drones.
“We need to act in an anti-bureaucratic way. This is the essence of a breakthrough in the war of technology. We are going to continue to put our bets on this, to work in this direction. Because technology can really save us,” he said, noting the shortage of artillery rounds Ukrainian troops were facing.
Overall, more than 300,000 drones of different types were contracted last year and more than 100,000 were sent to the front, he said, adding that the figures did not cover volunteer supplies which he said made a “significant contribution”.
“We removed taxes on UAV (unmanned aerial vehicle) components, simplified the contracting procedure, and the procedure for decommissioning,” he said.
“In other words, we took all the blockages that private sector companies were facing and addressed them in six months by passing all the necessary laws and resolutions.”
President Volodymyr Zelensky has set a target for Ukraine this year to produce one million First Person View (FPV) drones, which are cheap to make and weaponised in huge quantities by both sides on the front.