The UK Defence Secretary said there were individuals already in Ukraine "linked to the Russian state in ways that are not conventional".

Russia may already have personnel in place in Ukraine to assist with an invasion, the UK Government believes. Defence Secretary Ben Wallace told MPs there were individuals already in Ukraine "linked to the Russian state in ways that are not conventional" and "that should give cause for concern". He added: "We are becoming aware of a significant number of individuals that are assessed to be associated with Russian military advance force operations that currently are located in Ukraine."

It comes as concerns over a potential Russian invasion into Ukraine continue to increase. Moscow has massed an estimated 100,000 troops near the border with Ukraine. Mr Wallace told the Commons Foreign Affairs Committee that any incursion into Ukrainian territory would be unacceptable. He said: "Any crossing into Ukraine, whether small or large, would be viewed as a breach of that sovereignty, against international law and an invasion. "You can't be half-pregnant, you are either invading a country or you are not."

Hostilities have simmered for years but tensions are now escalating amid fears of a Russian invasion.

Kyiv, Ukraine – According to Washington, Russia has amassed more than 100,000 Russian soldiers on the border with Ukraine and in annexed Crimea in recent weeks.

This has stoked fears in Kyiv and the West that the Kremlin may start a new war with its neighbour and former province that chose to break away from Moscow’s political orbit.

Earlier this month, a top Ukrainian military expert told Al Jazeera that Russia could invade Ukraine as early as January, unleashing a “brief and victorious” war. But Russia denies it is planning an invasion. Moscow says it can move Russian troops wherever it wants and that any of its acts are defensive. Russian officials, including President Vladimir Putin, have for their part warned NATO against expanding eastwards. So, what is at the heart of the conflict that has been going on for more than seven years?

What is now Ukraine, Russia and neighbouring Belarus were born on the banks of the Dnieper River, almost 1,200 years ago in Kievan Rus, a medieval superpower that included a huge chunk of Eastern Europe. But Russians and Ukrainians parted ways linguistically, historically and, most importantly, politically.

Putin has, however, claimed repeatedly that Russians and Ukrainians are “one people”, part of the “Russian civilisation” that also includes neighbouring Belarus. Ukrainians reject his claims. Ukraine went through two revolutions in 2005 and 2014, both times rejecting Russia’s supremacy and seeking a path to join the European Union and NATO. Putin is particularly enraged by the prospect of NATO bases next to his borders and says Ukraine joining the US-led transatlantic alliance would mark the crossing of a red line.