Liz Truss has said it is “realistic” Russia could be pushed out of Crimea in comments that risk provoking Moscow amid the ongoing war in Ukraine.
Her words come just days after Russia's former president said any encroachment on the territory could lead to World War Three.
The Crimean peninsula is the disputed region in the south of Ukraine, which was seized by Russia in 2014 in a hostile move that Nato and the international community described as illegal.
Moscow disputes this and, on Monday, a close Putin ally warned that any encroachment on Crimea by Western forces would be "a declaration of war".
Since launching the invasion Russian troops have been pushed back from the capital, Kyiv, and other targets in the west of the country, and forced to focus on areas in the southern and eastern regions that neighbour the strategically important Crimea.
As a result, Russian forces have gained ground in the eastern Donbas region and on Saturday, pro-Russian forces claimed control of the city of Severodonetsk.
Watch: UK promises extra £1bn of military aid to Ukraine
Speaking as the UK announced on Thursday that it was almost doubling its military support to Ukraine with an extra £1bn to help with offensive operations, foreign secretary Liz Truss said that Russia should be ousted from all areas.
“All of Ukraine that has been invaded by Russia is illegally occupied," she told the BBC.
“And, ultimately, the Russians need to be pushed out of all of that territory, and certainly what we shouldn’t be doing as friends and allies… of Ukraine is implying that there are any trade-offs or any bits of Ukrainian territory that could be traded away or compromised on.”
Asked whether the government believes Russia can be pushed out of all of Ukraine within a foreseeable timeframe, she said: “It is realistic, and that is why we are supplying the extra lethal aid we’re supplying.”
Earlier this week, Dmitry Medvedev, who served as Russia's president between 2008 and 2012, told reporters any advancement on the peninsular in the south of Ukraine would be regarded as a provocation.
"For us, Crimea is a part of Russia. And that means forever. Any attempt to encroach on Crimea is a declaration of war against our country," Medvedev told the news website Argumenty i Fakty.
"And if this is done by a Nato member state, this means conflict with the entire North Atlantic alliance; a World War Three. A complete catastrophe."
Speaking at the Nato conference in Madrid, Boris Johnson on Thursday announced plans for the UK to spend 2.5% of GDP on defence by the end of this decade.
The PM said: “We need to invest for the long-term in vital capabilities like future combat air whilst simultaneously adapting to a more dangerous and more competitive world.
“The logical conclusion of the investments on which we propose to embark, these decisions, is that we’ll reach 2.5% of GDP on defence by the end of the decade.”
He also described the Nato alliance as “united” and in “robust health” as the Madrid summit drew to a close.
The military alliance announced on Monday it was boosting its high-readiness force nearly eightfold to 300,000 troops as part of its response to an “era of strategic competition”.
And on Wednesday the military alliance was poised to grow larger, after Nato formally invited Sweden and Finland to join.
Moscow has been clear that it opposes any chance for the alliance to get larger.
Last month, Putin said he would have "no problem" with the two nations joining — in a break away from previous rhetoric surrounding the topic — but warned an "expansion of military infrastructure into this territory would certainly provoke our response."