"We must ensure Ukraine wins this great fight for freedom," she said in a speech while visiting Prague.
Truss stated that Russia is acting not only against the Ukrainian people but also "freedom, sovereignty and the rule of law" in Western countries, TASS reported.
"If he (Russian President Vladimir Putin) succeeds in Ukraine, we will never feel safe again," she said, according to a copy of the speech released by the UK Foreign Office.
The foreign minister said appeasement is not an answer in the situation. "We must respond with strength. Appeasement cannot be the answer," she said.
On February 24 Putin announced a "special military operation" in response to a request for help by the heads of the Donbass republics. The US, EU, UK and some other countries retaliated by imposing sweeping sanctions on Russia.