Tensions between Ukraine and Russia have been increasing for months after the Kremlin massed some 100,000 troops near Ukraine's borders.

Western nations, including the United States and Britain, worry that the large build-up of Russian forces along Ukraine’s borders is preparation for an invasion as part of Moscow’s efforts to prevent Ukraine from ever joining the NATO Western security alliance.

Russia has repeatedly denied any plans to invade. However, in 2014 the Russian military seized Ukraine’s southern peninsula of Crimea and gave backing to separatist forces who took control of a swathe of eastern Ukraine.

The build-up of troops has been captured by satellite images, showing forces at bases encircling Ukraine’s northern, eastern and southern borders.

"Over the past month, our high-resolution satellite imagery has observed a number of new Russian deployments in Crimea as well as in several training areas in western Russia along the periphery of the Ukraine border," U.S.-based Maxar Technologies, a space technology company, said in a statement in late December.

The company identified BMP-series infantry fighting vehicles, tanks, self-propelled artillery and air defence equipment among the Russian forces newly arrived in the border region since October.

The following images show the Russian build-up at a few sites around Ukraine over the last two years:

While Ukraine's armed forces of more than 200,000 servicemen are less than a quarter the size of Russia's, they have been significantly boosted since 2014 by Western military aid, including supplies of U.S. Javelin anti-tank missiles and Turkish drones.

What would a Russian invasion look like?

Though Russia staunchly denies any plans to invade Ukraine, analysts still see its forces now well positioned to strike into the country.

Seth G. Jones and Philip G. Wasielewski of the Center for Strategic and International Studies (CSIS) said Russia's first move could include cyberattacks targeting Ukraine’s military command and control systems, public communications and electrical grids.

Air and missile strikes could next handicap Ukrainian air forces before Russian troops advance over hundreds of kilometres. According to CSIS, once Russian troops have invaded, there are different routes — many along rail lines — they could take based on Moscow’s objectives.

Russia likely will need to act soon or hold off on any westward move until the summer months. In March the area’s infamous ‘rasputitsa’ – or thawing of snow and ice – turns the land into mud and bog.

Jones explains that the muddy conditions could slow down armoured divisions and create bottlenecks, making those forces easier targets for anti-tank missiles.

How has the conflict reshaped Ukraine?

Ukraine has become the main flashpoint in Russia's relations with the West, but tensions began long before Crimea's 2014 annexation.

In 2004, Ukraine's presidential election led to mass protests, dubbed the “Orange Revolution.” After the Ukrainian Supreme Court voided the result and ordered a new vote, the country elected pro-European opposition leader Viktor Yushchenko. The protests and election were the first sign of Ukraine pulling away from Russia’s influence and flirting with closer alignment with Europe.

Protests again erupted on the streets in 2013, when then-president Viktor Yanukovich’s government suddenly announced he was suspending trade and association talks with the European Union and was opting instead to revive economic ties with Moscow. For months protesters organised mass rallies in Kiev, reaching 800,000-strong demonstrations by the end of 2013.

Why is Russia focused on Ukraine?

Russia's influence looms large in Ukraine, particularly in the urban, industrial east where Russian is the predominant language in many districts along the Ukrainian border as well as in Crimea in the south.

The lands of what are now Russia and Ukraine have been linked since the 9th century when Kyiv became the first capital of the Slavic state of Rus; in 988, under its ruler Grand Prince Vladimir, Rus was officially converted to Orthodox Christianity.

In June 2021, Russian President Vladimir Putin said Russians and Ukrainians were one people who shared a "single historic and spiritual space" and that the emergence of a "wall" between them in recent years was tragic. Kyiv rejected his argument.

Putin fears that Ukraine's growing ties with the West could turn it into a potential launchpad for NATO missiles targeted at Russia and could also inspire Russians with a pro-Western vision to oppose him. The prospect of NATO admitting Ukraine as a member or stationing weapons there that could strike Russia is a "red line" for Moscow.

Ukraine's westward pivot is evident in its exports and imports. Russia now accounts for just 8% of Ukraine’s international trade, while that with the European Union has climbed to 42%.

No one can be sure of Russia’s next move, but analysts agree that a war in Ukraine would be costly for both countries.

Western military analysts say Ukraine's army is better trained and equipped than in 2014, when Russia captured the Crimean peninsula without a fight, and is highly motivated to defend the country's heartland.

In the interim, the West is preparing tough sanctions should Russia invade while still pursuing last-ditch diplomatic talks to defuse the escalating conflict.