Foreign Secretary Liz Truss warned Russia not to invade Ukraine today amid mounting tensions on the border.
In her first major speech since being handed the job three months ago, the Cabinet Minister vowed to “stand with” Kiev as threats from Vladimir Putin’s regime grow.
“Any incursion by Russia into Ukraine would be a strategic mistake,” she told the Chatham House foreign affairs think tank.
Experts fear the Kremlin could invade by the middle of January.
The crisis is expected to dominate a G7 foreign ministers’ meeting hosted by Ms Truss in Liverpool this weekend.
“The problem here is Russian aggression, the problem is that Russia have built up troops and forces on the Ukrainian border,” she said.
“I am very clear that it would be a strategic mistake for Russia to stage an incursion into Ukraine.
“The issue is not Ukraine, the issue is for Russia to respect the sovereignty and the territorial integrity of Ukraine.”
Ms Truss, who later held talks with her Ukrainian counterpart over “closer strategic cooperation”, also spoke of the need to cut Europe’s reliance on Russian gas.
“We stand with Ukraine in supporting their security and defence and helping them become more energy independent,” she said.
"The EU relies on Russia for over 40% of its gas, and with some countries Russia has a complete monopoly of supply.
“If Russia gets its way, Europe will be increasingly hooked on its gas.
"We have to end this strategic dependency.”
Russian President Mr Putin said the Kremlin would send proposals to Washington DC next week to follow up his talks with US President Joe Biden on Ukraine on Tuesday.
He described the negotiations as open and constructive - stressing there was a possibility to continue the dialogue.
But he branded suggestions that Russia was planning to attack Ukraine "provocative".
As well as discussing the growing crisis, Ms Truss will use this weekend’s G7 summit of foreign ministers from the US, France, Germany, Canada, Italy and Japan to urge fellow ministers to “stop fighting about the past”.
Outlining her aims for the meeting, she told Chatham House: “My message will be clear - it’s time to get on the front foot and join us in advancing the frontiers of freedom.
“It is time to dump the baggage, ditch the introspection and step forward, proud of who we are and what we stand for, ready to shape the world anew.
“Let’s stop fighting about the past; let’s start fighting for the future.
“By championing our ideas, building our influence and inspiring others with our cause, we can forge ahead as a global network of liberty.”
Speaking after talks with Ukrainian counterpart Dmytro Kuleba, Ms Truss said: “The situation facing Ukraine today is one we have seen time and again from the Kremlin playbook.
“Russia is trying to destabilise its democratic neighbours in a futile attempt to exert control over them.
“The UK, together with our NATO and European partners, will continue to support Ukraine’s sovereignty and territorial integrity.
“We stand with our allies to defend and advance the frontiers of freedom.
“We call on Russia to de-escalate tensions and abide by its international commitments including reporting troop movements and returning to the table for serious talks.”
