When Russia invaded in February, Western powers feared an onslaught that would see Ukrainian forces crumble within days, but Kyiv has dashed Russian hopes for a quick win.

Crucial to that success has been Ukraine's determination to struggle to the bitter end, epitomised by the weeks-long resistance first in the southeastern city of Mariupol, and now in Severodonetsk.

"The strategy has been -- overall -- very effective," Ivan Klyszcz, a researcher at Estonia's University of Tartu, told AFP, crediting it with forcing Russia to leave the north after failing to seize Kyiv.

The current stalemate on the eastern front is also the result of Ukraine's ferocious defence of its cities, Klyszcz added.

"Every time Russian troops were slowed down in a city, it stopped them from having a dynamic, from gaining territory or from quickly seizing a city," a French military source who asked not to be named told AFP.

Ukrainian troops held out under siege for weeks in the Azovstal steelworks in Mariupol while the rest of the city lay in ruins, before finally surrendering to Russian forces last month.

"The siege of Mariupol compelled Russia to allocate substantial forces" to try to take control of the port city, said William Schneider, a researcher at the Washington-based Hudson Institute.

As a result, Moscow was forced to delay the deployment of more than 12 Russian battalions to the eastern Donbas region, Schneider told AFP.

-'War of attrition' -

This week, Lugansk governor Sergiy Gaiday said around 10,000 civilians were trapped in the industrial city of Severodonetsk, where fighting with Russia has raged for weeks.

Capturing the city would allow Russian forces to advance further into the Donbas region, which Russia appears to want to annex, Schneider said, upping the importance of resisting the onslaught.

Gustav Gressel, a researcher for the European Council on Foreign Relations, said it made sense for Ukraine to force much of the fighting to take place in cities.

"Urban terrain favours the defender ... if you can force the enemy to fight there, you stand better chances," Gressel said.

"This is a long war of attrition. Small, incremental Russian gains are not the issue, but rather who erodes the other faster?" he added.

In addition to the strategic benefits of die-hard resistance, a never-give-up attitude also crucially keeps morale up and dissuades Ukrainian forces from throwing in the towel.

"Even if it is desperate, it is a way of guaranteeing the consolidation of the units, more and more made up of young soldiers or volunteers who joined the war late and need to be encouraged by example," the French military source said.

-Fatigue-

Ukrainians' image abroad may also benefit from a hero-like reputation as they come across as "martyrs", especially in Mariupol, the military source added.

After Mariupol and Severodonetsk, observers are on the lookout for the next fighting hotspot.

"It's hard to predict ... but the Kherson area has witnessed many Ukrainian gains and may become a contested city in the coming days and weeks," Klyszcz said.

But questions remain over the long-term sustainability of such an approach, as Moscow continues its onslaught.

"As Ukraine's resources, war materiel and manpower begin to dwindle, the strategy risks becoming unviable," Klyszcz warned.

Kyiv recently admitted that around 100 Ukrainians were dying a day, and 500 injured -- although the numbers may be even higher on the Russian side.

And the British Ministry of Defence said last week that some Ukrainians were deserting their armed forces.

"We are beginning to see soldiers who are dropping out as a result of the pressure, fatigue, and intensity of the firepower befalling them," the French military source said.

dla-ech/js/pvh

Twitter

SCHNEIDER ELECTRIC