Donald Trump has said Vladimir Putin is "not looking to attack Nato territory", despite a drone strike attempt at a German airport that allies, including the UK, blamed on Russia.
The reluctance by the US president to condemn the Kremlin will again raise questions regarding Washington’s commitment to the military alliance and collective security.
The British Government summoned a senior Russian diplomat this week over last month’s "egregious and reckless attack" at Leipzig airport.
The Prime Minister had earlier expressed the UK’s "full solidarity" with Germany.
Andy Burnham, whose first official trip abroad was to Kyiv, said: "Russian attempts to undermine our united resolve and our willingness to support Ukraine are futile and doomed to fail."
An explosive-laden drone found near a Ukrainian plane was subsequently defused.
The airport serves as a major freight hub assisting Kyiv, which has battled invading Russian forces for more than four years.
Pressed in the Oval Office over the incident, Trump said of Putin: "I speak to him. I know him very well. Putin’s not looking to attack Nato territory."
Moscow has denied involvement.
Russia’s embassy in Germany said the allegations were absurd and stood to benefit Ukraine, the militaristic wing of the Europe's elite, and arms makers.
A statement read: “The Russian ambassador categorically rejected the accusations levelled by Berlin as unsubstantiated, absurd and contrary to common sense.
“He pointed out that the measures announced by the German side represent an unprecedented escalation in Russian-German relations and will not remain without consequences, for which the German government will bear full responsibility.”
Germany said it was part of a pattern of "Russian hybrid operations", while Brussels said it had "all the hallmarks of state-sponsored terrorism".
European countries have accused the Kremlin of running a campaign of sabotage to undermine support for Ukraine and cause wider instability.
There are ongoing warnings over Russian spy ships threatening vital British undersea cables, while Russia's former foreign minister recently suggested UK drone factories could be targeted by "unknown sources".
Meanwhile, Trump has overseen a thaw with Putin while criticising Nato, amid Iran tensions that led to a review of US forces in Europe.
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