• Russian President Vladimir Putin is being watched closely by experts and officials.
• They fear that Russia is planning to invade Ukraine.
• Tens of thousands of Russian troops have reportedly gathered at the border with Ukraine.
• Russia invaded and annexed Crimea, a part of Ukraine, in 2014.

President Vladimir Putin is being watched closely by experts and officials who fear Russia might be planning a military escalation with its neighbor Ukraine.

Tens of thousands of Russian troops have reportedly gathered at the border with Ukraine, and experts fear Russia could be about to stage a repeat of its 2014 invasion and annexation of the Ukrainian peninsula of Crimea, which prompted global outrage and sanctions on Moscow.

"We all should be very worried, to be honest, I do share this assessment," Michal Baranowski, director and senior fellow at the German Marshall Fund's Warsaw Office, told CNBC when asked if Russia could be about to embark upon military action against Ukraine, describing Russia's highly tense relationship with Ukraine as being a conflict "under the threshold of war."

"This assessment is shared by many here in Warsaw and in Washington, D.C.," he told CNBC's Hadley Gamble on Wednesday, adding "we are seeing very significant buildup in threats on the border with Ukraine. So it's really a key moment for the West to step up pressure against Putin."

Last week, U.S. officials reportedly warned their European counterparts that Russia could be weighing a potential invasion of Ukraine. The Defense Ministry in Kyiv said in early November that about 90,000 troops were massing on the border while Ukrainian President Volodymyr Zelenskiy said last week that there were nearly 100,000 Russian soldiers at the border, Reuters reported.

For his part, when asked whether Russia is plotting a military invasion in Ukraine, Putin dismissed such a notion as "alarmist" in an interview with Rossiya 1 last weekend.

Russia has also sought to play down the movements of its troops, with Putin's press secretary, Dmitry Peskov, saying last week that "the movement of troops on our territory shouldn't be a cause for anyone's concern," The Associated Press reported. CNBC has contacted Russia's Defense Ministry for further comment.

Concerns over Russia's possible next move when it comes to Ukraine, which used to be part of the Soviet Union before its dissolution in 1991, come against a wider backdrop of deteriorating relations between Russia and its allies on one side — and Europe and the U.S., on the other.

Tensions have emerged on a number of fronts from energy and political meddling to cyberwarfare and migrants, with Russia accused of helping Belarus to orchestrate a growing migrant crisis on the EU's doorstep.

Russia expert Timothy Ash, a senior emerging markets sovereign strategist at BlueBay Asset Management, noted Tuesday that it "feels like Putin is bracing for war with Ukraine. He has the motive, opportunity and weapon."

His motive, Ash said, was that "he wants Ukraine, as he never accepted its loss in 91' with the collapse of the USSR, and sees Ukraine as central to his vision of Greater Russia."

The opportunity, Ash said, was that "the West is weak, divided and lacks focus. Biden is focused on China, Europe on gas, migrants and the Balkans. Ukraine is politically weak."

And Putin's potential weapon? "Well, an invasion force of several hundred thousand in/around Ukraine, including forces already in Donbas and Crimea. Add in gas, migrants, political intrigue, cyber, space," Ash said.

International concerns

U.S. Secretary of State Antony Blinken said last week that Washington was concerned by reports of "unusual Russian military activity" near Russia's border with Ukraine, warning that should Russia "commit further aggressive acts against Ukraine, we are committed, and Germany is committed, to taking appropriate action." He did not state what that action might entail.

European leaders have continued to voice their concerns this week.

Germany's and France's respective foreign ministers, Heiko Maas and Jean-Yves Le Drian, issued a statement Tuesday pledging their "unwavering support for the independence, sovereignty and territorial integrity of Ukraine." 

"Against the background of renewed concerns about Russian troop and material movements near Ukraine, we call on Russia to exercise restraint and to provide transparent information about its military activities. Any new attempt to undermine Ukraine's territorial integrity would have dire consequences," the ministers said.

Meanwhile, NATO's secretary general, Jens Stoltenberg, said Monday on Twitter that the military alliance was "closely monitoring the large and unusual concentrations of Russian forces close to Ukraine's borders. We call on Russia to be transparent, prevent escalation & reduce tensions."

Meanwhile, France's president, Emmanuel Macron, told Putin on Monday that his country was ready to defend Ukraine's territorial integrity and sovereignty. But just how far the EU and U.S. would go to defend Ukraine is uncertain.

Last week, U.S. President Joe Biden and European Commission President Ursula von der Leyen discussed Ukraine, and said they "fully supported Ukraine's territorial integrity" but did not mention how far they would go to protect the country.

Ukraine has said the West must send a strong message to Moscow to not take any aggressive action.

"If we continue to have strong Western partners standing by us and they take resolute actions, this will help us to prevent the war and to prevent the bloodshed and this is what we are now focused on," Ukraine's foreign minister, Dmytro Kuleba, told the BBC's "Today" program Wednesday.

Tensions on other fronts

Russia has been accused of helping to stir up another crisis on the border between Belarus and Poland, where masses of mainly Middle Eastern migrants have gathered in a bid to enter the EU.

Putin's ally Belarus has been accused of "weaponizing" migrants and contriving the migrant crisis (essentially by inviting migrants to the country in the knowledge that they will then try to enter the EU via Poland) in order to destabilize the bloc and to distract from Russia's troop buildup.

Belarus denies it has engineered the migrant crisis and Russia denies any involvement, with Putin's press secretary telling reporters last week that "Russia — like other countries — is trying to get involved in resolving the situation."

Another source of underlying tension is energy, with Russia accused of orchestrating an energy price crisis in Europe in recent months by withholding supplies as it awaits the regulatory greenlight for the contentious Nord Stream 2 gas pipeline which will send its gas supplies to Europe, bypassing Ukraine.

German regulators suspended the certification process of the pipeline this week, saying its operating company needed to be compliant with German law before it could approve the pipeline. Gas prices soared in Europe on Tuesday as a result.

The pipeline is controversial in Europe with Poland and Ukraine saying the project threatens Europe's energy security (Ukraine will also lose out on vital gas transit fees it earns when gas supplies enter Europe via its own pipelines). The U.S. (which competes with Russia for a share of Europe's gas market) has also poured scorn on the pipeline.

Nord Stream 2 certainly poses a problem for Europe; on one hand it relies on Russia's gas imports (around 40% of the EU's natural gas imports come from Russia) but it has vowed to protect Ukraine, a country that has ambitions to join the EU, much to Russia's annoyance.

U.K. Prime Minister Boris Johnson warned earlier this week that the EU needed to choose between "mainlining" Russian gas and supporting Ukraine.