Western capitals have warned of a possible Russian air assault on Ukraine with intelligence reports indicating Moscow is preparing to throw its jets and helicopters into the war to support a stuttering land offensive.

The fear had prompted allies to prioritise rapid shipments of air defence assets and artillery ammunition to Kyiv, western officials said, to respond to the shift in approach by Moscow as the almost year-long war enters a new phase.

Intelligence shared among Nato allies shows that Russia is amassing fixed-wing and rotary aircraft on the border with Ukraine, according to two officials briefed on its contents.

In meetings with allied countries supporting Ukraine on Tuesday, US defence secretary Lloyd Austin highlighted the threat of Russia’s significant remaining air force.

“He was very clear that we have a short window of time to help the Ukrainians to prepare for an offensive and that they had some pretty specific needs,” a senior US administration official said.

“The Russian land forces are pretty depleted so it’s the best indication that they will turn this into an air fight. If the Ukrainians are going to survive . . . they need to have as many air defence capabilities and as much ammunition . . . as possible,” the official added.

Since the first weeks of the war, following President Vladimir Putin’s full-scale invasion on February 24 last year, Russia has used its sizeable air forces sparingly, relying instead on long-range missiles, artillery and land-based troops.

Western analysts had speculated that this could have been due to a fear in Moscow that Ukraine’s air defence systems posed a threat to Russian aircraft, or because its fleet was in poor condition.

But intelligence assessments indicate that Russia’s air force is “actually quite preserved”, a senior Nato diplomat said.

“More than 80 per cent probably is safe and available . . . So we are expecting that they’re preparing to launch an air campaign and they’ve been trying to [disable] Ukrainian air defences with attacks,” the diplomat said.

The warning comes as some 50 western allies used a meeting at Nato’s headquarters on Tuesday to lay out additional pledges of military equipment for Ukraine. The US is expected to announce another assistance package later this week, which will mostly include air defence and ammunition, according to people familiar with the matter.

Ukrainian and Nato officials have in recent days warned of ammunition shortages, with western capitals growing concerned at the effect of strained defence supply chains and dwindling domestic stockpiles.

These immediate needs trumped longer-term requests for materiel such as fighter jets or quicker shipments of western battle tanks, officials said, given Russia’s new offensive, which Nato said on Monday had already begun.

“​​What Ukraine tells us that they really need is ammunition and additional air defence capabilities,” a western official said.

Germany’s defence minister Boris Pistorius said on Tuesday that the arms producer Rheinmetall would restart production of ammunition for Gepard anti-aircraft guns as part of the new push to support Kyiv.

The Gepard, which the German military decommissioned in 2010, has proved to be an effective weapon for the Ukrainian military to shoot down drones and low-flying missiles. But ammunition has been running low, and a recent effort by Berlin to persuade Switzerland and Brazil to replenish it from their own stocks ended in failure.

“We will quickly start our own production of Gepard ammunition at Rheinmetall,” Pistorius said. “I am very happy we have been able to guarantee the delivery of this important part of air defence.”

While Ukrainian president Volodymyr Zelenskyy last week rekindled a debate among allies about whether or not to send fighter jets, a US official said these were less important than ammunition in the near term as Ukraine looked to maintain an edge.

“What we really have to concentrate on now is Ukraine’s ability to defend the air and they’ll do that through air defence artillery, equipped with the proper ammunition. Fighter jets are not and will not be as capable against the Russian air force as an integrated air defence system,” said a US official.

While no decisions were expected on Tuesday, the issue of supplying fighter jets was to be discussed, two people briefed on the preparations said. President Joe Biden has said the US will not send F-16s but American officials concede they are likely to provide Ukraine with more sophisticated air power as the war drags on — or at least give a green light to the transfer of F-16s from other powers.

The allies’ meeting could also include talks about a greater number of more direct contracts between Kyiv and western defence companies, brokered and financed by allied countries, two officials said. That would streamline the flow of supplies from production line to deployment, while also acknowledging that western militaries have little more to give from their reserves.

Ukraine and Russia are engaged in fierce fighting around the town of Bakhmut in eastern Ukraine, which US officials believe could fall to Moscow this week.

The town is of little strategic importance, but its loss would be a symbolic blow to Kyiv after months of fighting. Russia has also stepped up attacks elsewhere in the Donbas and renewed its air and missile attacks on other areas of Ukraine. Moscow has 300,000 troops in Ukraine as it gears up for its new offensive.

Increasingly Russia is using the sheer size of its forces as an advantage but has suffered steep casualties. US officials estimate about 200,000 Russian troops have been killed or wounded.

The US, Germany, Poland and the UK last month agreed to provide main battle tanks. The American heavy armour will take months to arrive, while German and British tanks could arrive in the spring, with Pistorius on Tuesday expressing frustration at the “not exactly breathtaking” pace of deliveries.

Additional reporting by Laura Pitel in Berlin