U.S. officials have, reportedly, warned their European counterparts that a Russian invasion of Ukrainian territory may be imminent. The warning comes less than a decade after Russia forcefully annexed Crimea despite international protests.

A report from Daily Telegraph claimed that European officials were warned that Russia might launch a military assault in Ukraine. The assumption is based on a massive build-up of Russian troops near the Russia-Ukraine border.

Last week, Ukraine's defense ministry said they had detected the build-up, which they estimate to be around 90,000 Russian ground troops. The scene near the border is reminiscent of the prequel to the Russian invasion of Crimea, which resulted in the annexation of the territory.

U.S. secretary of state Antony Blinken said Wednesday that Russia's intentions remain to be unclear. However, based on its previous actions, Blinken said there is a huge possibility that the country may make an attempt to annex more territory. Blinken said there is concern that Russia may attempt a repeat of its actions in 2014.

On Thursday, Russian officials claimed that one of its jet fighters had intercepted a British spy plane flying near Crimea. A Boeing RC-135 aircraft reportedly flew near the Black Sea peninsula, which is still considered to be within Ukrainian territory. The plane was forced to alter its path after a Russian Sukhoi SU-30 fighter jet intercepted it.

Tensions in the region have escalated in recent months as the U.S. and its allies have increased their military activity in response to the build-up of Russian forces near the Ukrainian border.

Russia's defense ministry said it had detected four planes belonging to NATO countries operating near the Black Sea on Thursday. This includes the British plane that was supposedly forced to change its flight path. Moscow claimed that two Russian warships were also watching two U.S. vessels in the area.

Russian President Vladimir Putin criticized the U.S. and its allies for increasing their presence in the area. Putin said the presence of Western forces in the area is "provocative."

Meanwhile, the U.S. claims that it is merely monitoring the situation in the Ukrainian border in response to the presence of Russian troops. The assistant secretary of state for Europe and Eurasia, Karen Donfried, reiterated that the U.S. has made it "very clear" that it will be standing with Ukraine in case Russia does try to invade.

"Any time we see unusual Russian military activity near Ukraine we make clear that any escalatory or aggressive action is of great concern to the United States," Donfried said.