Washington rejected Warsaw's proposal to transfer Polish MiG-29 fighter jets to Ukraine through a US military base in Germany as the West seeks for ways to help Kyiv fight Russian invasion but fears being sucked into a war with nuclear-armed Moscow.
Here are NATO allies' considerations on providing fighter jets to Ukraine, which was attacked from the air, sea and land on 24 Feb.
Moscow has targeted Ukrainian cities, sending some 2 million civilians fleeing and triggering more EU sanctions on Russian banks and trade, as well as blacklisting of officials and oligarchs.
Poland is the largest ex-communist state in NATO and the European Union, and has a long history of fighting Russia and seeking to integrate with the West. It now sits on the eastern border of both blocs and neighbours Ukraine, with which it has many cultural and historical links.
But Poland refused to go solo in providing jets to Ukraine, fearing it would be exposed to Russian retaliation without all of the NATO alliance rallying behind it.
"Such a serious decision as supplying planes must be unanimous and unequivocally made by the whole North Atlantic alliance," said Polish Prime Minister Mateusz Morawiecki.
Warsaw has warned previously about NATO's blind spot known as the Suwalki Gap, a 100-kilometre-long (60-mile) border running through farmland and woods on the Polish-Lithuanian frontier, the only thing that separates Russia's ally Belarus from Moscow's Baltic enclave of Kaliningrad.
Germany handed over 22 MiG-29 to Poland in 2003-04 and, under a resale clause, Poland would need to get Berlin's approval to pass them on to Ukraine.
Russia's invasion of Ukraine has already forced Berlin to rethink its long-declared ban on exporting arms to war zones, a policy designed to break with the country's historical legacy of waging wars on Europe in the 20th century.
Germany announced it would supply anti-tank weapons and anti-aircraft weapons from its military stocks to Kyiv, as well as authorising Estonia passing on old East German howitzers and the Netherlands offering German-produced RPGs.
While it has moved a long way from its initial offer of 5,000 helmets to Ukraine in late January, Germany is also keen to avoid moves that Russian President Vladimir Putin would see as provocation meriting military response.
Germany hosts the Ramstein air base, the US military's gateway to Europe and part of the biggest US military community overseas, with around 50,000 service members, civilian employees and families living there.
The 30 NATO countries are legally bound to protect each other if one comes under attack. But the allies are wary of Putin's nuclear threat and have already refused Ukraine's calls to set up a no-fly zone to help protect it from Russian missiles and warplanes.
"We are not part of this conflict," NATO head Jens Stoltenberg said last week.
Two NATO sources told Reuters this week that providing fighter jets to Ukraine would also risk dragging the Western military alliance into a full-on military confrontation with Russia.
"You could change the colours on the jets but you can't hide it from Russia," said one of the sources, who spoke under condition of anonymity.
"The question is whether we are willing to accept Russia firing off missiles towards Tallinn or Riga, or somewhere in the Nordics, or in Poland. There is no political appetite for that."