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Russia has amassed more than 100,000 troops near Ukraine's border in recent weeks, according to U.S. estimates, raising fears from Western and Ukrainian intelligence officials that an invasion could be imminent.

As frantic diplomatic efforts are made to avert war, analysts are warning that Russia's military poses an immediate threat to Ukraine.

But if an invasion were to occur, it is not clear where it would begin. Russia has created pressure points on three sides of Ukraine -- in Crimea to the south, on the Russian side of the two countries' border, and in Belarus to the north.

Here are the three fronts Ukraine and the West are watching, and the recent Russian movements detected in each.

EASTERN UKRAINE

Most attention has been paid to the breakaway regions of Donetsk and Luhansk, where Ukrainian forces and Russian-backed separatists have been in conflict since 2014.

The foremost assumption of those watching Russian movements is that Moscow could boost the military might it already possesses in the region, therefore making eastern Ukraine the easiest position from which to launch an invasion.

shows that a large base at Yelnya, which held Russian tanks, artillery and other armor, has been largely emptied, with the equipment apparently being moved much closer to the frontier in recent days.

Large amounts of weaponry were moved to the base late in 2021 before disappearing -- including some 700 tanks, infantry fighting vehicles and ballistic missile launchers. Social media videos since show some of that equipment on trains and roads much further south in the Bryansk region, which is close to Ukraine. The armor and vehicles are identifiably from the same units that had pre-positioned at Yelnya.

Stephen Wood, senior director at satellite imagery company Maxar, told CNN: "It looks to me like a considerable amount of the vehicles [tanks, self-propelled artillery and other support vehicles] have departed from the northeastern vehicle park; additional armored vehicles departed from the more central vehicle park."

Meanwhile, heightened activity in the Kursk and Belgorod Oblasts, which border northeastern Ukraine, has

"We are seeing a massive influx of vehicles and personnel in Kursk," Konrad Muzyka, an expert in tracking military movements with Rochan Consulting, warned on Twitter.

Phillip Karber of the Potomac Foundation in Washington, who has also studied Russian troop movements in detail, told CNN this month: "Russia's strongest offensive formation -- the First Guards Tank Army, which is normally stationed in the Moscow area -- has moved south 400 kilometers (250 miles) and is assembling in the optimum area for a rapid armored offensive on the Khursk-Kyiv invasion route."

BELARUS

Concerns have also grown over a vast build-up of Russian troops in Belarus, a country closely allied to Moscow that could provide another way into Ukraine.

Russia and Belarus began on Thursday, the size and timing of which has sparked fears in the West.

Moscow's deployment into Belarus is believed to be its biggest there since the Cold War, with "an expected 30,000 combat troops, Spetsnaz special operation forces, fighter jets including SU-35, Iskander dual-capable missiles and S-400 air defense systems," NATO Secretary General Jens Stoltenberg said on February 3.

It is also the largest exercise the Belarusian armed forces have conducted at any time of year, according to the Center for Strategic and International Studies (CSIS). Russia's Ministry of Defense claimed the purpose of the drills, called "Allied Resolve-2022," included repelling "external aggression."

Some fear the build-up points to a Russian plan to surge towards Kyiv from the north. One European diplomat told CNN earlier this month that the massing of forces is a "big, big worry," noting this would be the missing piece that Moscow would need to launch a quick attack on the Ukrainian capital.

The joint drills would also provide cover for a flanking movement through Belarus and into northern Ukraine,

And satellite images released by Maxar appear to show that at several locations in Belarus. The deployments are likely linked to the joint exercises, but other photographs show camps being established close to the border with Ukraine, hundreds of miles from where the exercises are taking place.

However, if Russia were to focus on the Belarusian border as its entry point to Ukraine, the route would be fraught with difficulties.

Russian soldiers would have to negotiate the Pinsk Marshes, also known as the Pripet Marshes, one of Europe's largest wetlands, which straddles the border between Belarus and Ukraine -- a stretching across 104,000 square miles.

That region impeded Nazi forces during Operation Barbarossa, Germany's doomed invasion of the Soviet Union, in 1941.

According to the Institute for the Study of War, "the marshes can be difficult, in some places likely impossible, for mechanized forces to traverse when wet."

CRIMEA

The peninsula annexed by Russia in 2014 would provide a natural staging ground for any new operation, but it is unclear whether Moscow would attempt to launch a move into Ukraine from Crimea.

A has been observed by Maxar, which assesses that more than 550 troop tents and hundreds of vehicles have arrived north of the Crimean capital, Simferopol.

Then a new deployment was identified by Maxar for the first time Thursday near the town of Slavne on the northwest coast of Crimea, including armored vehicles.

Those new deployments were observed on the same day that several Russian warships arrived in Sevastopol, Crimea's main port. The Russian Defense Ministry posted images Thursday of six large amphibious landing ships at the port.

Ukraine's Navy responded that "Russia continues to militarize the Black Sea Region, transferring additional landing ships to put pressure on Ukraine and the world."

Ukraine's naval forces "are ready for development of any scenarios and provocations, to defend the country from the sea," it added.

Any move into southern Ukraine could be aided by troops in Transnistria, the Russian-supported breakaway region of Moldova, where build-up has also been reported.

CSIS analysts say Russian troops could attempt a coup de main on Odessa, a Ukrainian port city the northwest of Crimea, by "sailing its amphibious ships straight into Odessa's port and moving directly into the city."

It calls such a move "a high gain but also a high-risk operation." Odessa is a well-populated city and urban combat there would favor those defending it, while Russian forces would need to eliminate Ukraine's air defenses and then link up with troops arriving from the east of the country.