Downing Street describes Russia’s actions as “worrying”.
The UK has pledged to offer “unwavering support” to Ukraine amid concerns about a buildup of Russian troops on the country’s border.
The Kremlin has denied claims that it was preparing for an invasion after the Ukrainian Defense Ministry reported that around 90,000 Russian soldiers were gathering in the area.
However, Downing Street described the situation as “worrying” and said Britain would assist Ukraine in maintaining its borders.
Russia annexed the Crimean peninsula from Ukraine in 2014 and there is an ongoing conflict between the two nations over the territory.
Asked about the Russian build-up of troops, Boris Johnson’s official spokesman said on Monday: “We are seeing a worrying situation on the border.
“We continue to steadfastly support the territorial integrity of Ukraine and will continue to support it in the face of Russian hostility.”
It came when NATO Secretary General Jens Stoltenberg said that the Western military alliance was “with clear eyes” about the challenge that Russia’s “substantial” military build-up poses.
“We are seeing an unusual concentration of troops and we know that Russia was ready to use this type of military capability to take aggressive action against Ukraine,” he said.
US Secretary of State Antony Blinken said last week it would be a “grave mistake for Russia to repeat what it did in 2014” when it captured Crimea from Kiev.
Defense Minister Ben Wallace stressed that the UK has a “very, very operational combat group in Estonia that can be moved in a matter of hours” and suggested that it would serve as a deterrent against Russia.
Meanwhile, the UK accused the authoritarian President of Belarus, Alexander Lukashenko, of “heinous” attempt to create a migrant crisis in order to undermine European unity.
Mr Johnson said the UK stands “shoulder to shoulder” with its European allies as EU foreign ministers consider new sanctions against the Minsk regime.
Thousands of migrants, mainly from the Middle East, are trapped at the Belarusian border with Poland, where the authorities have denied them entry into the EU.
Lukashenko, a close ally of Russian President Vladimir Putin, has been accused of deliberately encouraging migrants to cross EU borders in retaliation for the sanctions Brussels imposed in response to its repressive rule.
Foreign Minister Liz Truss called on Putin on Sunday to intervene to stop the development in the region, which she described as a “shameful manufactured migrant crisis”.