Airmen and civilians palletise ammunition bound for Ukraine during a foreign military sales mission in Delaware. File/AP

The US numbers alone are mounting: more than 12,000 weapons designed to defeat armored vehicles, some 1,400 shoulder-fired Stinger missiles to shoot down aircraft, and more than 50 million rounds of ammunition, among many other things. Dozens of other nations are adding to the totals.

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The Biden administration is preparing yet another, more diverse, package of military support possibly totaling $750 million to be announced in coming days, a senior US defence official said on Tuesday. The official spoke on condition of anonymity to discuss plans not yet publicly announced. The additional aid is a sign that the administration intends to continue expanding its support for Ukraine's war effort.

These armaments have helped an under-gunned Ukrainian military defy predictions that it would be quickly overrun by Russia. They explain in part why Vladimir Putin’s army gave up, at least for now, its attempt to capture Kyiv, the capital, and has narrowed its focus to battling for eastern and southern Ukraine.

Pallets of ammunition bound for Ukraine are being loaded at Dover Air Force Base, Delaware. AP

US officials and analysts offer numerous explanations for why the Russians have had so little success interdicting Western arms moving overland from neighboring countries, including Poland. Among the likely reasons: Russia's failure to win full control of Ukraine's skies has limited its use of air power. Also, the Russians have struggled to deliver weapons and supplies to their own troops in Ukraine.

Some say Moscow's problem begins at home.

"The short answer to the question is that they are an epically incompetent army badly led from the very top,” said James Stavridis, a retired US Navy admiral who was the top NATO commander in Europe from 2009 to 2013.

The Russians also face practical obstacles. Robert G. Bell, a longtime NATO official and now a professor at the Sam Nunn School of International Affairs at Georgia Tech University, said the shipments lend themselves to being hidden or disguised in ways that can make them elusive to the Russians — "short of having a network of espionage on the scene” to pinpoint the convoys' movements.

Western weaponry pouring into Ukraine helped blunt Russia's initial offensive in Ukraine's Donbas region.

"It’s not as easy to stop this assistance flow as it might seem,” said Stephen Biddle, a professor of international and public affairs at Columbia University. "Things like ammunition and shoulder-fired missiles can be transported in trucks that look just like any other commercial truck. And the trucks carrying the munitions the Russians want to interdict are just a small part of a much larger flow of goods and commerce moving around in Poland and Ukraine and across the border.

"So the Russians have to find the needle in this very big haystack to destroy the weapons and ammo they’re after and not waste scarce munitions on trucks full of printer paper or baby diapers or who knows what.”