The defence secretary, Ben Wallace, said it is highly unlikely that Britain or its allies will send troops to defend Ukraine if it is invaded by neighbouring Russia.
US intelligence claims that Russia has about 70,000 troops stationed near the border of Ukraine and has begun planning for a possible invasion as soon as early next year.
On 16 November, Wallace and the Ukrainian defence minister, Oleksii Yuriyovych Reznikov, issued a joint statement saying they were concerned by Russia's military buildup.
The United Kingdom stands shoulder to shoulder with the people of Ukraine and will continue its longstanding determination to support them, the statement said.
Wallace said in an interview with the Spectator that Ukraine is not a member of Nato, so it is highly unlikely that anyone will send troops into Ukraine to challenge Russia. We shouldn't kid people we wouldn't. In comments carried by the Times before the interview, the Ukrainians are aware of that, he added.
Asked if that meant Ukraine was on its own, Wallace said: We can all help with capacity building but in some way Ukraine is not in Nato and that is why we are diplomatically saying to Putin don't do that. It comes days after Boris Johnson told the Russian leader Vladimir Putin, there would be significant consequences for Russia if it invaded Ukraine.
The prime minister spoke to Putin by phone on Monday to reassert the UK's commitment to Ukraine's territorial integrity and warn that any destabilising action by Moscow would be a strategic mistake. The Kremlin denied preparing an invasion and accused Kyiv of stoking tensions in the region by deploying new weapons.
Since 2014, the two countries have been at odds when Russian forces have annexed the Crimean peninsula and supported a separatist rebellion in eastern Ukraine.