Why Russian troops are massing near Ukraine's 'fortress city'

Russian forces are reportedly gathering across the border from Ukraine's "fortress city" - but why? After pushing Ukrainian troops out of Kursk, the Russian region they had occupied parts of for many months, what's left of the 50,000-strong Russian force is positioned just across the border from Kharkiv.

Ukraine war's largest prisoner swap - latest updates A major push along all or part of the frontline by Moscow's troops has been expected for some time but has not yet fully materialised, analysts say. It's believed Russia has a "four-month window" to break through Ukrainian forces before the weather begins to turn and dwindling stocks of Soviet tanks possibly start to run low.

Where are the massing Russian troops - and are elite soldiers among them? Kremlin troops have been gathering on the other side of the border near Kharkiv in northeast Ukraine, according to a senior Ukrainian military figure. "The enemy is trying to pull its personnel closer to the line of combat contact and conduct at least some assault actions," Andriy Pomahaibus, chief of staff of the 13th Operational Brigade, said this week.

"In general, they are not succeeding." Nonetheless, he said there is "clear preparation for active assault actions by the enemy". Sky News military analyst Michael Clarke says the accumulation of troops comes off the back of the Russian operation to push Ukraine out of the pocket of the Kursk region they had occupied, which is not far from Kharkiv.

"Now they have Kursk back the argument is: will they keep going?" Some of Russia's most experienced soldiers - including from the elite VDV airborne unit - were moved to reinforce the Kursk campaign, and could still be among the gathered troops. Prof Clarke adds: "If they have left those units there, that would suggest they want them to spearhead something else.

"If those units turn up back around Pokrovsk (in Donetsk) that would then mean they are not about to build up a major strategic attack near Kharkiv." Read more:Ex-Ukrainian politician shot dead on school runRussia launches 'heaviest drone attack' since start of war Is Russia preparing to attack Kharkiv? Prof Clarke says it's possible Russia is preparing for a big push near the border cities of Kharkiv and Sumy. This could either be a direct attack on one of the cities - a tough task given how well defended they are - or an attempt to capture much of the surrounding area Ukraine liberated in Autumn 2022.

Located just 20 miles from the border with Russia, Kharkiv is Ukraine's second-largest city with a pre-2022 population of more than a million. It is regarded as a "fortress" and was awarded the distinction Hero City of Ukraine for its resistance during the opening months of the 2022 invasion.

However, Prof Clarke doesn't think Russia has the resources for a big push at Kharkiv or Sumy this summer, after the Kremlin instead opted to try to attack along wide stretches of the frontline. "I think the Russians basically used up the forces they might have otherwise used up for a strategic offensive," he says.

This, he says, caused Ukraine to use up its reserves to counter the Russian attacks. "Both sides have sacrificed the possibility of a strategic offensive for this ongoing battle of attrition." 'Four-month window' for Russia to make a breakthrough Dr Jack Watling, a military expert from the RUSI thinktank, argues Russia will likely "soft launch" its offensive rather than going for a rapid manoeuvre by large mechanised units.

"The Russians lack the force quality to operate in this way," he says. Instead, the summer offensive will likely see a "steady increase in the number and scale of assaults across a broadening area.