The rout of Russian troops in north-eastern Ukraine has raised hopes among Nato allies that a well-supplied army could liberate even more territory, strengthening demands to rapidly expand the supply of western arms to Kyiv.

The Ukrainian advance has emboldened officials from the US, UK and some of the more hawkish members of the EU, several officials told the Financial Times, citing private conversations over the weekend. While the counter-offensive was under way, western defence ministers met in Germany to discuss maintaining weapons supplies.

“The tone has shifted, without a doubt,” said a senior European diplomat. “You won’t really hear anyone talking against more weapons now, just a chorus of supporters and one or two staying silent.”

“It is 100 per cent true that more weapons mean more Ukrainian territory,” said a second official. “And less blood, less tears.”

But western intelligence and defence officials cautioned that the advance would not necessarily be repeated all along the war’s front line. They added that while it would probably force Moscow to reassess its strategic aims it was unlikely to herald a collapse of Vladimir Putin’s army.

“I’m watching this closely,” a senior western intelligence official said of the Ukrainian advance. “Cautiously optimistic.”

The Ukrainian advance, which began early last week, is the biggest military setback for Russia since it was forced to retreat from the north of the country in March after five weeks of failing to encircle Kyiv.

Ukraine’s defence minister Oleksii Reznikov told the FT that while the advance had gone “better than expected”, Kyiv had to be careful not to be overconfident and allow its front line supply lines to become overstretched and risk a Russian counter-attack. “We have to be worried,” he said.

Moscow has claimed that the significant shift in the war’s front line was the result of a decision to “regroup” and move forces further south to repel Ukrainian attacks in the occupied Kherson region.

Konrad Muzyka, director of Poland-based defence analytics company Rochan Consulting, said it was now possible to imagine Ukrainian forces advancing to positions they held before Putin launched his full-scale invasion on February 24 if western military support was maintained and there was no full-scale mobilisation of the Russian population.

Before the invasion, large parts of the Donbas region were controlled by Kremlin proxies; Crimea was annexed by Russia in 2014.

“We are probably now entering the third stage of this war, where Ukraine has the initiative and Ukraine gets to decide what the focus will now be,” said Muzyka.

“It does not mean that Ukraine will achieve all that it wants but that now it gets to choose where the war goes,” he added. “It is only going to get harder from here.”

The US, UK and other Nato allies have pledged to supply Ukraine with more than $16bn in physical weapons, but have only delivered half that so far, according to publicly available data collated by the Kiel Institute for the World Economy, a German think-tank.

Weapons shipments have been sporadic in recent months, western officials have admitted, due to logistical challenges and concerns among the allies that their own stocks are running low.

Security officials and analysts say the recent advances, which involve the use of western arms, have strengthened the case for stepping up the scale of shipments and the rate of delivery. In particular, they highlight the use of high-precision long-range missiles such as the US-made Himars system.

“The Himars are clearly making a huge difference,” said the western intelligence official. “As has the allied training and weapons.”

“If Himars had been there from day one, then the conflict would have been very different,” said Muzyka, adding that the missile systems would have probably “annihilated” Russia’s immobile and exposed supply lines during their failed assault on Kyiv.

Lithuania’s foreign minister Gabrielius Landsbergis said on Sunday that allies who had previously dragged their feet on weapons supplies were guilty of “appeasement”. He said the success around Kharkiv had proved “beyond doubt that Ukraine could have thrown Russia out months ago if they had been provided with the necessary equipment from day one”.

“Putin’s genocidal plan to wipe Ukraine off the map has failed,” he said. “He is in no position to negotiate. The war must end with his unconditional surrender.”

But other officials cautioned that, despite the shifting front line, the war is likely to last for months. Many western capitals believe Ukraine’s military strength should be bolstered to improve the country’s position on the battlefield and thus any eventual peace talks.

“Make no mistake . . . we have to be prepared for the long haul and be ready to provide support to Ukraine for as long as it takes,” Jens Stoltenberg, Nato secretary-general, said on Friday as the scale of Ukraine’s advance was becoming clear. “Wars are by nature unpredictable. And we know that Russia has a lot of military capabilities.”