RUSSIA is reportedly building up a 50,000-strong force alongside the border of Kharkiv amid fears of a fresh summer offensive to attack the city.

With the US stepping back from direct discussions to mediate a peace agreement and Vladimir Putin delaying ceasefire negotiations, specialists are worried that a significant Russian offensive might be set into motion in the coming weeks.

In April, the Ukrainian military reported that the Kremlin was gathering forces in preparation for a new attack on Kharkiv, Ukraine’s second-largest city.

Meanwhile, President Volodymyr Zelensky also said that Moscow has been attempting to gain ground in the border regions of Sumy in the northeast.

Military analysts believe he is trying to press home his advantage and capture more Ukrainian land.

They warn that Putin only has a “four-month window” to get a breakthrough in Ukraine this year.

This move could signal the onset of Russia’s summer campaign, aiming at the border city of Kharkiv — known as Ukraine’s “fortress” city — which displayed strong resistance at the onset of the invasion.

It comes as today Putin launched his largest air attack of the war – blanketing Ukraine with hundreds of drones and missiles.

Andrii Pomahaibus, the chief of staff of Ukraine’s 13th National Guard Khartiia Brigade, said this week: “The enemy is trying to pull its personnel closer to the line of combat contact and conduct at least some assault actions.”

Reports of troops massing near Kharkiv came after Putin successfully flushed out the Ukrainians from Kursk, which Kyiv captured last year.

Putin is said to have deployed 50,000 soldiers – including troops from North Korea – to regain the land he lost.

And some of the most experienced Russian soldiers were reportedly part of the Kursk counteroffensive – not too far from Kharkiv’s backyard.

Professor Michael Clarke, a renowned military analyst, told Sky News: “If they have left those units there, that would suggest they want them to spearhead something else.”

Defence experts say Moscow could – at least – try to get back the land they lost back in 2022 when Ukraine launched its counteroffensive, if not launch direct attacks on the cities.

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”, he added. “Indeed, there are indications this process has already started.”

This is because even though Russia is hellbent on continuing the bloody war, it does not have the resources to sustain a large-scale ground offensive.

Professor Clarke says Russia only has a “four-month window” to put together a final assault before its already-dwindling reserves dry up.

He said: “I think they must know this is their last year of build-up before they reach a plateau [in tank productions].”

Dr Watling added: “Russian stockpiles of legacy Soviet equipment, from tanks and infantry fighting vehicles, to artillery pieces, will be running out between now and mid-autumn, such that Russia’s ability to replace losses will be entirely dependent on what it can produce from scratch.”

Putin’s moves in recent weeks suggest he has no intention of giving up on the war just yet.

And with the US finally pulling out of direct peace efforts, the Russian tyrant would certainly look to capitalise on the situation and leverage it to gain more from a future peace deal.

Even though Putin himself suggested direct peace talks, he failed to show up at Istanbul to meet Ukraine’s boss Zelensky.

And while diplomats from both countries did manage to strike a largely ceremonial deal to swap 1000 orinsons from each side, they failed to make concrete gains towards a permanent solution to the war.

This is because Putin is still hellbent on making maximalist demands while claiming his forces can fight for years.

Meanwhile, Kremlin spokesman Dmitry Peskov said there was no agreement on where the next round of talks would take place.

European leaders, including Prime Minister Sir Keir Starmer ,have accused Russian President Vladimir Putin of dragging his feet in peace efforts.

It comes as Russia intensified its air attack on Ukraine in the past 48 hours.

Nato scrambled fighter jets after the Russian forces last night launched 367 missiles and drones in the largest single attack since the start of the war in 2022.

Yuriy Ihnat, a spokesperson for Ukraine’s Air Force, said Moscow used a mix of 69 ballistic and cruise missiles and 298 drones, including Iranian-designed Shahed drones.

Sounds of explosions boomed throughout the night as Ukrainian air defence forces worked to thwart the Russian onslaught.

Twpo nights ago, the Russian forces launched a massive air assault on Kyiv with 250 drones and 14 Iskander-M ballistic missiles.

Apartment blocks were set ablaze by the barrage as residents in the Ukrainian capital fled to underground shelters amid the sound of anti-aircraft fire.

Explosions rocked the city during a seven-hour raid as fireballs lit up the skyline long into the early hours.