Britain has said that it wants Russia to de-escalate tensions along the Ukrainian border, where Moscow has amassed more than 90,000 troops.
"You've seen the increasing deployment on the border and clearly we need to address that," a spokesperson for Prime Minister Boris Johnson told reporters on December 6. "We think that Russia needs to step back and de-escalate the tensions there."
The comments came a day before a virtual meeting between U.S. President Joe Biden and Russian President Vladimir Putin.
Russia's troop buildup, as well as Ukraine's efforts to join the NATO alliance, are expected to top the agenda of the recently scheduled meeting.
U.S. intelligence reports have suggested that Russia could be preparing to invade Ukraine as early as 2022. Russia seized Ukraine's Crimean Peninsula in 2014 and backs separatists in eastern Ukraine who have waged a bloody war against Kyiv that began the same year.
Moscow has demanded written guarantees that Ukraine will not be allowed to join NATO, calling such a scenario a "red line." The Kremlin has also expressed concerns about Western weapons supplies to Kyiv, as well as military drills in international waters of the Black Sea.
NATO and Ukraine, a former Soviet republic, have had friendly relations since the country gained its independence in the early 1990s. The two have deepened their cooperation since Russia's military actions in Ukraine in 2014.
Ukraine adopted legislation in 2017 reiterating its aim to join NATO as a strategic objective. However, President Volodymyr Zelenskiy this summer expressed frustration over the issue, calling on Biden to give Kyiv either a "yes" or "no" on mapping out a plan for Ukraine to enter the alliance.
The United States has provided more than $2.5 billion in security assistance to Ukraine since 2014, and both Washington and NATO have offered assurances of their unwavering support for Ukraine in the face of Russian aggression.
Regarding Moscow's ultimatums, Biden has said that he will not accept "anyone's red line."
Biden and Putin have met as presidents in person once, in Geneva in June. They last spoke by telephone in July.