A Russian incursion into Ukraine would violate the basic freedoms and sovereignty of the defence secretary after a visit to Scandinavia.
After a second warning to Moscow last week, Ben Wallace said there would be consequences of Russian aggression towards Ukraine.
On Monday he shot across the bows, telling an event in London that Britain would stand up to bullying no matter how far away the conflict.
Tensions on the Ukrainian border continue to be fraught, with the US suggesting Moscow is preparing for a false flag operation in order to spark an incursion into neighbouring territory.
The new US intelligence was unveiled after little progress was made in the talks between the Kremlin and Washington and its western allies to get rid of the escalating crisis.
Foreign secretary Liz Truss, who appears to back up the US intelligence, accused Russia of waging a disinformation campaign that would destabilise and justify an invasion of Ukraine, and called for a de-escalation of the military presence.
Wallace met Nordic partners this week, including Finland, which shares a border with Russia, amid the standoff.
The Ministry of Defence said he met with ministers in Sweden, Finland and Norway to discuss Russia's continued aggression and military build-up on Ukraine's border, with about 100,000 troops amassed at the divide.
The defence secretary said that the UK and our Nordic partners are united in our approach to uphold European security.
My discussions this week have focused on deepening bilateral relations, shared security and the consequences of Russian aggression towards Ukraine.
Our discussions were clear that a Russian invasion of Ukraine would be a strategic mistake, violating the most basic freedoms and sovereignty.
Britain and the Nordic countries have a long and shared history.
Our European neighbours and allies are vital partners as we work together to defend our common values, counter shared threats and build resilience in our neighbourhood. Labour said the threats facing Ukraine must be met with strength and resolve by Nato and Europe after the shadow defence secretary, John Healey, visited the country this week.
Healey said that western allies must work hard to maintain their deterrent pressure on Russia and stand by Ukraine.
According to the MoD, Wallace met his Swedish counterpart, defence minister Peter Hultqvist, before he went to Finland to hold talks with president Sauli Niinist, defense minister Antti Kaikkonen.
During the third leg of his trip, the defence secretary met his Norwegian counterpart Odd Roger Enoksen.