Vladimir Putin’s invasion could be defeated partly by Ukrainian forces’ tactics of fighting “delaying battles” and then withdrawing troops before they are encircled, British defence chiefs said on Thursday.
They stressed that Ukrainian soldiers were still holding their defensive positions in Lysychansk after the withdrawal from its twin city of Severodonetsk in the eastern Luhansk province.
Mr Putin’s army is not believed to be planning a major new crossing of the Siverskyi Donets, between the two cities, given the heavy losses suffered at other failed river crossings.
However, Russian troops are reported to be making “limited progress” in their attempts to encircle Ukrainian fighters in the northern part of the Donetsk province of the eastern Donbas region.
In its latest intelligence update, the Ministry of Defence in London said: “Ukrainian forces continue to hold their positions in the city of Lysychansk following their withdrawal from Sieverodonetsk.
“Russian forces continue to pursue an approach of creeping envelopment from the Popasna direction, removing the need to force a major new crossing of the Siverskyi Donets River in this sector.
“Current ground combat is likely focused around the Lyschansk oil refinery, 10km (six miles) south-west of the city centre.”
Mr Putin’s military seized full control of Severodonetsk at the weekend.
The British military chiefs added: “At the operational level, Russian forces continue to make limited progress as they attempt to encircle Ukrainian defenders in northern Donetsk Oblast via advances from Izium.
“It is highly likely that Ukrainian forces’ ability to continue fighting delaying battles, and then withdraw troops in good order before they are encircled, will continue to be a key factor in the outcome of the campaign.”
Britain, America and other allies are fighting an information war against Mr Putin’s regime, so the intelligence reports they make public need to be treated with caution.
However, the Kremlin’s claims on many fronts, including the killing of civilians and on war crimes, are often totally contradicted by numerous reports from the war zone.
Russia pressed on with its offensive in eastern Ukraine on Thursday after Nato branded Moscow the biggest "direct threat" to Western security and agreed plans to modernise Kyiv's beleaguered armed forces.
Ukrainian authorities said they were trying to evacuate residents from the frontline eastern city of Lysychansk, the focus of Russia's attacks where about 15,000 people remained under relentless shelling.
"Fighting is going on all the time. The Russians are constantly on the offensive. There is no let-up," regional Governor Serhiy Gaidai told Ukrainian television.
Mr Putin’s forces are trying to encircle Lysychansk as they seek to capture the Donbas region, which comprises Donetsk and Luhansk provinces where Moscow-backed separatists have been fighting Ukraine’s army since 2014.
At a summit on Wednesday dominated by Russia's invasion of Ukraine and the geopolitical upheaval it has caused, Nato invited Sweden and Finland to join and pledged a seven-fold increase to more than 300,000 from 2023 in combat forces on high alert.
In reaction, Mr Putin said Russia would respond in kind if Nato set up infrastructure in Finland and Sweden after they join the US-led military alliance.
However, US President Joe Biden announced more land, sea and air force deployments across Europe from Spain in the west to Romania and Poland bordering Ukraine.
These included a permanent army headquarters with accompanying battalion in Poland - the first full-time U.S. deployment on NATO's eastern fringes.
"President Putin's war against Ukraine has shattered peace in Europe and has created the biggest security crisis in Europe since the Second World War," said Nato Secretary-General Jens Stoltenberg.
"Nato has responded with strength and unity."
Britain said it would provide another £1 billion ($1.2 billion) of military support to Ukraine, including air defence systems, uncrewed aerial vehicles and new electronic warfare equipment.
But Ukrainian President Volodymyr Zelensky once again told Nato that Ukrainian forces needed more weapons and money, and faster, to erode Russia’s huge edge in artillery and missile firepower, and said Moscow’s ambitions did not stop at Ukraine.
As the 30 national Nato leaders were meeting in Madrid, Russian forces intensified attacks in Ukraine, including missile strikes and shelling on the southern Mykolaiv region close to front lines and the Black Sea.
The mayor of Mykolaiv city said a Russian missile had killed at least five people in a residential building there, while Moscow said its forces had hit what it called a training base for foreign mercenaries in the region.
Attacks on the northeastern city of Kharkiv continued, Ukraine's military said, with shelling from tanks, mortars and missiles.
Video showed police and emergency workers examining the destroyed remains of a factory and damaged residential buildings in Kharkiv on Wednesday.
“Where, why and what they (Russians) targeted is unclear, yet. We are figuring out where they aimed because there are no military units in that area,” said Volodymyr Tymoshko, chief of the Kharkiv region’s police.
The Russian invasion that began on February 24 has destroyed cities, killed thousands and sent millions fleeing. Russia says it is pursuing a "special military operation" to rid Ukraine of dangerous nationalists. Ukraine and the West accuse Russia of an unprovoked, imperial-style land grab.